What is the overall scale and visual style you're aiming for?
How "Far back" is the camera going to be? How far in do you want to be able to pull the camera, and what kind of detail do you want to do?
If you pull back far enough in a realistic visual style, then infantry animations are almost vague shadows and blurs, and the most visible part of their animation would be muzzle flashes and the explosions of grenades and such. (And the UI element can be more of an icon centred over their location simply for visibility and usability.)
For a less realistic style, then honestly I'm perfectly happy with infantry being little circles with an arrow pointing off them. They could even be rendered with the 'vague blurry hard to see' styles, as if you're watching them from some high altitude UAV trying to look through heat shimmer, with some kind of Augmented Reality overlay on them that some movies have been suggesting since the 80's.
Give this a read through and see if anything fits your style.
https://en.wikipedia.org/wiki/NATO_Joint_Military_Symbology
Well designed infantry make a strategy game far more interesting than if you leave them out. They're far too interesting from a mechanics point of view in my mind thanks to their ability to dig in and hold on to a space that simply doesn't work with vehicles. And they get even more interesting if you include resupply mechanics, and having to cut off/defend supply lines.
Personally I think that infantry work best if they're treated as entire units of reasonable size and properly scaled. If you control individual tanks to move around the battlefield, then consider platoon sized infantry units. If a 'unit' of tanks has multiple tanks that drive around in formations and support each other, then control companies of infantry.
Also keep in mind your scale and who you want your player to feel like. If I'm supposed to be high ranking and having overall control over a battle, then controlling an individual soldier and having to directly click on specific locations to get the most out of them is just awkward in my mind. I much rather click an infantry unit and issue orders along the lines of "hold this area", or "ready to attack this zone", and have the AI handle specific details. However the 'handling the details' also goes for vehicles. If I select a heavy tank and a bunch of fast scouting vehicles and tell them all to move somewhere as a group, then the default control input should be that the scouts spread out around the tank and dart ahead from cover to cover. If the scouts race off miles ahead to die before the tank can get close enough to shoot, then there is a problem. Equally so if the scouts all slowly trundle along at the same speed as the tank and allow themselves to get shot easily when in the open. (Basically, give me a solid AI that controls small details, and the larger scale controls to fine-tune what the AI is doing. Do I want the AI moving up carefully with multiple units, leap frogging them so that one moves forward while the other digs, then the second moves forward after the first has dug in? Do I want them all to just rush headlong and get to a battle zone as quickly as possible and take the risk of ambush? Do I want them to move forward carefully, halting their advance and forming a battle line on contact with the enemy? Etc.)
Consider having infantry as more flexible health and damage output, but scale with different weapon types.
- A full strength infantry unit throws out a lot of lead and can cover a lot of angles at one time. As it gets damaged it will slowly cover fewer angles but still keep relatively high fire power in those (because they'll keep manning their machine guns/mortars)
- A tank might become slowed or tracked as it takes damage, or might have its main weapon knocked out. Vehicles become more binary or 'glass cannon' in design, either fully functioning or knocked out, with little or no variables in between.
Other important things to consider: What are the resources available, and what is the impact of the locals? Are there civilians? Do we care about them? Caring about civilians and civilian properties is important in how we deal with something like an infantry company holding a small village - Do we need lots of our own infantry to move in and clear the area, or does enemy infantry holding a village just mean that our heavy artillery units will be unavailable to fire at other things for a few minutes while they reduce the village to a nice mostly level parking lot?
Then there is also the asymmetric warfare aspect to consider. One side might have heavy artillery that could level the village while the other side doesn't. However levelling the village means that you now have destroyed all of your infantry cover in that part of the map while you take out the enemy - Is this going to come back to bite you? Maybe your 'militia conscript' infantry units are no match for the enemy elite 'guerrilla mercenaries' in the open field, and they'll be able to gain a 'dig in' bonus in the fields around the former village while your soldiers would only get a similar bonus if the village was still standing.
