Hello all! I'm new to the forum and I'm glad to have found a lot of interesting discussions/topics! 

Quick intro, I'm currently in school for Independent (indie) Video Game Design, on my last semester and the job search will start in less than 4 months (I'm nervous to say the least). I've learned a lot in school and I'm proud to say that I can make a decent game independently and market it properly. The problem is that I can do all of this, but I don't specialise in anything specific. I'm pretty good at modeling (but definitely not a pro, can only make simple clean models), okay at scripting, design isn't my strength but a big interest and I'm pretty okay at UI/UX but definitely not proficient at all.

I can't say I specialise in any of the above fields and I know that specialising in something is important in order to have a consistent portfolio and finding a job. 

Should I focus on specialising on a specific field in the next 4 months (practice 24/7) in order to sell myself to employers or should I practice everything and sell myself as a Jack-of-all-trades? I really want to get a designer job as I enjoy writing GDDs and discussing design during Pre-Production but my Rational Design knowledge is weak and I've never been considered a designer in all my previous projects (always was responsible for art or UI). 

Look at job specifications. They tell you exactly what is needed to get a job.

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The problem is that I can do all of this, but I don't specialise in anything specific. I'm pretty good at modeling (but definitely not a pro, can only make simple clean models), okay at scripting, design isn't my strength but a big interest and I'm pretty okay at UI/UX but definitely not proficient at all.

This is pretty normal for people who get into indie game development. In order to create an indie game, you have to learn everything yourself in order to create a game by yourself.
 

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I can't say I specialise in any of the above fields and I know that specialising in something is important in order to have a consistent portfolio and finding a job. 

Usually this is the case for jobs looking for already-experienced people in certain areas.
As you will be fresh out of school, and it sounds like you haven't had an internship, jobs looking to hire will likely notice that you're going to need on-the-job training. Any decent company would take the time to train you so you'll become an asset to them.
 

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Should I focus on specialising on a specific field in the next 4 months (practice 24/7) in order to sell myself to employers or should I practice everything and sell myself as a Jack-of-all-trades?

I say why not?
Working on several skills at once will, obviously, take much more time than learning one skill at a time.
It may make you a jack-of-all-trades, but take this for example:

• You are hired into a game company with others who are familiar with certain tools, but they are not advanced in them.
• One person asks "hey, I can't figure out how to ... can you help me?" but you are only experienced slightly in all fields, so you are unable to help them.
• You notice you are unable to help with many tasks, because you don't have advanced knowledge in any of the fields.

If you had learned one particular skill before moving onto the next, you might have been able to help more effectively.

Usually it is important to have really learned at least one thing. But the more things you try to learn sequentially, the more you may forget how to do previous tasks. For example, you learn 3D modeling. Then you move to audio. Then you move to coding. By this time, you may have forgotten certain key shortcuts from when you were 3D modeling.

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Overall, I don't think it will hurt your chances of finding a job as they will provide on-the-job training, but I do believe it could be beneficial to learn at least one skill in-depth

4 hours ago, abe97 said:

Should I focus on specialising on a specific field ...  in all my previous projects (always was responsible for art or UI). 

Then unless you create a portfolio that proves something different, hirers will see you as an art/UI person.

4 hours ago, abe97 said:

I really want to get a designer job

Well, entry-level game design jobs are extremely rare. Make a design portfolio, but be willing to start in another field. You can always level up to designer, once your foot is in the door.