Asymmetric 4X, the player starts in the middle surrounded by various aliens the goal being conquest of the galaxy (50% let's say) and survival against late game invasion of aliens from another galaxy spawning around the edges of the galaxy. Purely singleplayer game.

I have implemented most of the things, the player starts with 50 planets in the middle, AI attack, etc. Everything is fine except the game flow. If I made the AI too strong it crushes the player, if I made it too weak the player conquers everyone without effort, if I made them balanced... there is a stalemate :D

So, it's kind of "overbalanced" I need somehow to make it so the game can progress (player can win but with effort).

I'm not sure how to make it... The AI produces a static number of units per turn (partially based on number of planets), the player can produce units highly depending on population size (planets are needed to house population) so the player's production capabilities grow over time, with a soft cap (bureaucracy eat up most of the income after reaching 20 billion population), also the payer has research.

Somehow, I need to make it so the player can upset the balance. Or change the flow of the game? I don't know...

If the player is better than alien, they win, else they lose.

You can add random events and hidden knowledge to create opportunities to change the tide of battle, if that fits the game. So if alien is winning, maybe player can strike a weakness to prevent defeat.

That would make the "uncertainty" region of who will win, bigger, since theres more unknowns. So game remains interesting for longer, when its not just the first moments of gameplay that determine the winner.

Then, the problem of momentum. Once a side is winning, what prevents them from pushing through all the way?

First, just make everything slow enough, so it takes some time. Maybe taking over a planet is not instantaneous, maybe you need to build infrastructure before being able to push further.

Second, let the battlefront oscillate. Like a spring with ever increasing energy, until it finally exceeds a limit in either end and one side is victorious. You push hard, both sides lose resources, but you are left with weakened force, allowing alien to push back, repeat.

One way to implement that would be to give limited offensive power to both sides, that takes time to regenerate, and isnt sufficient to wipe out the entire enemy in one go. Basically, you 'spend' your offensive capability to hurt the enemy somewhere, but have to go defensive after that, giving enemy an opportunity to do the same.

Additionally, there should be multiple ways of dealing with threats/aliens,

Destroying their fleet, destroying their planets(make it not too-easy to do both simultaneously) making a treaty(less resources, but bigger threat later)

scorched earth(destroying they could otherwise conquer, in such a way they can no longer move towards the player's other planets)

Things like these mainly exist to make all games differently, instead of there being one "road to victory" there are multiple roads,

they should not all hold the same chance of achieving victory, but they should all be the best chance of achieving victory in different situations that may/will occur.

If the player is better than alien, they win, else they lose.

It's not exactly like that.

Typically, in 4X, everyone starts with 1 planet. The AI gets early bonuses so instantly becomes stronger than the player. The player lie low in that phase. Then, since the player is smarter eventually there comes a point when sides are equal (mid game). After a while the player becomes stronger and starts extermination of the AI (late game).

In my case it works different. You basicly start at the mid game (a lot of fleets guarding planets, 10-50 planets under your control, most planets in the galaxy already colonized). So, it start in a quite balanced state... which is not best for the gameflow :)

Once a side is winning, what prevents them from pushing through all the way?

Asymmetric nature of the game. AI does not have the goal of crushing the player (or at least not all AIs). So, much powerful races can exist alongside the player without breaking the game.

One way to implement that would be to give limited offensive power to both sides, that takes time to regenerate, and isnt sufficient to wipe out the entire enemy in one go. Basically, you 'spend' your offensive capability to hurt the enemy somewhere, but have to go defensive after that, giving enemy an opportunity to do the same.

That's quite similar to how it works now. It results in a stalemate on strategic level (you conquer one planet, you lose one planet and after 100 turns nothing really changed).

I have the problem with the stalemate.

With the weaker AI do you think that the game is too easy or do you know that the game is to easy? How long does the game actually take with the weaker AI? Have you laid out some sort of road map of just what sort of progress you expect a typical player to experience?

What if you were to implement both the easy and hard AIs and have them switch on and off periodically (possibly with some mechanism to alter the period)? Maybe this represents shifts in leadership or other internal events of some kind.

This is where I would start on a 4x game, before all the little details about population and food, or what weapons the ships would have. Everything else should be built to serve whatever you come up with to give the game good flow.

When I play civilization on an easy setting, I get to a point where I can tell that I'm far in the lead and that I'm going to win, but still have several hours of game play until I win. This can ruin the state of flow for me even more than a stalemate. A game that allows bigger swings in battle can help with this for both the winning and losing players. If you're winning, you could take advantage of your lead to defeat your opponent quickly, while a losing player could take a risk to quickly even the playing field. This might make it more challenging to achieve a balance so a player that wins one war doesn't move on to take over the whole world/galaxy every time, but I think has more potential for an exciting game. When you do have a well balanced war, instead of feeling bored by the stale mate, you would be excited to know that either player can tip the war in their favor at any moment.

Also, in civilization a stalemate in wars can still work because there are other victory conditions that you'll eventually get to after you've researched the end game tech. An end condition like the space race allows players to always be making progress towards something, instead of simply gaining and losing ground.

Everything is fine except the game flow. If I made the AI too strong it crushes the player, if I made it too weak the player conquers everyone without effort, if I made them balanced... there is a stalemate

Maybe have the AI strength oscillate over time. For a while the alien leader is a bit of a doofus and it's relatively easy to make gains against them. Then for a while the alien leader is a strong tactician and your progress is impeded or even set back.

So, to trivialize the suggestion: the player has a "rage bar" when it fills up the player can fire it up and all ships get +100% damage for 30 turns :) AND/OR AI has -50% damage for 20 turns then +50% to damage for 20 turns and the cycle repeats? :)

If you've managed to scale the difficulty, why not make it scale-able in-game? Essentially cheat it a little. Set the initial difficulty to your "easy" level. Create some kind of non-player-visible "scorecard" or something that gives you a good indication whether or not the player is losing. If it appears the player is getting his ass kicked, scale the difficulty down by say 5%. If he's still getting beat, go further down. Do the opposite if he's winning. You can keep the flow slightly in the player's favor, maybe ramping it up a bit occasionally to offer a challenge.

Most 4x games I've played completely cheat the AI resources. Try isolating a planet late-game and watch them somehow continue to churn out dreadnaughts and super-weapons despite apparently having no access to resources. If you don't do a little bit of cheating, the end-game is generally anti-climatic. After you've conquered 60% of the map, the remaining 40% is just clean-up. Depending on game-length, the final clean-up can take hours and hours.

If it becomes too obvious, you can either make it more subtle or add randomized -10% or +10% based on nothing to make it feel like surges. Unless the illusion is broken by a sole planet churning out dreadnoughts, the player should think that it's all a result of their own actions.

If it appears the player is getting his ass kicked, scale the difficulty down by say 5%. If he's still getting beat, go further down. Do the opposite if he's winning. You can keep the flow slightly in the player's favor, maybe ramping it up a bit occasionally to offer a challenge.

This is absolutely no go! There were a thread here recently (that I can't find right now) about adjusting difficulty to player skill. If I remember correctly the conclusion was that it infuriate players more than unbalanced difficulty. It's the player who has to be in charge or changing difficulty below or above his skill.

So, to trivialize the suggestion: the player has a "rage bar" when it fills up the player can fire it up and all ships get +100% damage for 30 turns AND/OR AI has -50% damage for 20 turns then +50% to damage for 20 turns and the cycle repeats?

This or (semi)random events that can strike the balance. Something like sabotage resulting in much lower defense for X turns for otherwise large, stable and strong empire (be it player or AI)