I'm thinking of getting a career with a lot of free time so that I could meditate and chant . I live in a third-world country. Also, my saturn is in gemini and in the third-house which caused me to have problems with speech, manual dexterity, and writing (sad face). My natal saturn also caused me to temporarily skip college for now. Which led me to think that any prospect in writing / art related jobs would be harder to achieve. So I was thinking maybe I could become a petroleum engineer or biologist? But I am unsure right now.
(I have mars and uranus in the 12th house, and 3 planets in the 9th).

There is not one job on Earth that has "free time" as a perk. By definition, a job means you're there to do something and provide value. You need to be honest with your intentions as you will get what you ask for.

You could technically build a self-sufficient business sacrificing 5 years of your time. The idea would have to be well-thought and well-executed. Refer to people like MJ DeMarco and others to understand how a successful business works. Then, after those 5 years, you could sell for high profit and keep yourself afloat with proper knowledge of investments. Lately, I always suggest this option because we live in a corrupted world where employment is very unbalanced and not appropriately retributed.

This is ideal in the current enemy society. However, you also need to think of how you want to contribute to your race. It's very important to make a contribution that impacts millions. Sure, your future business may do that without interaction from you once that milestone is reached, but that doesn't stop you from finding other ways to contribute to society by creating value in other industries.

I appreciate you want to spend a lot of your time in developing spiritual power and expanding your awareness. However, that's all not there is to life. That is just one pillar of your existence. The Ancients identified 5 aims of life: moksha, dharma, kama (relationships), artha (finance) and arogya (health). Spiritual advancement, which is moksha, is only one of them and is related and supported from all others. All the aims are, in fact, interdependent. This is also why there are 12 houses in your life and not just 1.

You may be oriented toward developing one of your pillars like you mentioned because Mars in the 12th house puts the majority of your focus into that but you also need to be realistic with yourself and try to understand how life works and imbalances like that will not bode well your spiritual advancement.

More free time is something you build up to and it needs to be built carefully without creating an excess of it which is ultimately detrimental. Being a petroleum engineer and biologist won't give you that immediately nor at any stage unless you manage to invent something revolutionary at some point or build your own successful business in those fields in a way that it allows you to interact with it less after a few years of hard work.

Not to mention both professions you mention require you to enrol in higher education and complete your degrees at a good standard. Most professional biologists need a master degree at the very minimum and, in some cases, even a PhD. With writing and art, instead, it's not really necessary but they're mostly freelance jobs if you want to have the freedom to choose what to write and what artwork to reproduce. Otherwise, you will have to get into employment that often requires long hours beyond the 9-5 window. Things like being a concept artist or a video game write or tv show writer, etc.

I suggest you research the market. Depending on what "third-world" country you are in, you may consider temporarily relocated somewhere else such as Japan, Singapore or India if you're Asian. The worst place possible to relocate is the USA because of how expensive healthcare and education are. You definitely do not want to be tens or hundreds of dollars in debit just because you were ill-advised enough to go to university there. Similarly, Australia and the United Kingdom are overly expensive countries when it comes to higher education and the debt you built is usually not worth it unless you come from a family when you don't need to worry about student loans and stuff like that.

For art and writing, you don't really higher education anyway. If you want to go into employment with this, rather than anything else, what impresses the most a prospective employer is how you can provide value to them, with practical examples. For example, with writing you can write fanfictions on many websites online and build your portfolio up like that. You can also write for popular apps like Episode who gives writer the chance to create their own work for other users to play (for free). Unpaid experience will allow you to showcase your value much more than someone who has no experience and just empty words. For art, I'm sure there are similar gigs and artists on our forum will be able to tell you about it.

If you are still undecided about your career, look at your 10th, 6th and 2nd house. What sign is on the cusp, what is the ruler, where is the ruler placed, what are the aspects to both the ruler and the cusp... Find answers to these questions, then blend the information. That should give you some clarity about many possibilities.

In Britain, there are jobs that are guaranteed 9-5 (or similar) with no overtime and no weekends. If your country or a developed country of your own race has something similar, this is a good starting point to develop key soft skills and it also leaves you with enough free time to take care of your spiritual needs, of your body, and so on. If you have no children, that is. If you have a partner in the future who works full-time in a well-paid job or has a successful business, you also have the alternative of being in a part-time job which makes for greater work-life balance.