@zivusmutavi you will get as many opinions as there are members on here :-)
First remember that we are not fixing things for profit here. We are doing this for a more altruistic reason. We do want to keep our devices out of the landfill, decrease e-waste and want you to safe your money by repairing instead of replacing. Here is my personal mission statement on that About Repair is War on Entropy - iFixit
Now my answer to you and not the answer for anyone else here will be this. Apple was once the product that has most bang for the buck, products were repairable and schematics in general were available. That has now gone by the wayside. I'd stay away from anything Post 2020/2022 Prior stuff is still okay. Lenovo and Dell are once more getting pretty popular, and motherboard schematics are a 50/50 for availability. Cellphone and TV repair is not a lucrative market. There is no way to get a profit from those. Cost for parts and the initial repair will most often be higher than buying new. People will stay away from that. Also, most "broken" stuff has had previous repair attempts. Most of the time it's more of a butchery which makes it even more difficult to repair.
If I would start off new, I'd be looking at Drone repair as well as specialize in LCD display repair. Most "damaged "screens" are actually experiencing issues with the driver boards and would be fixable with the right equipment.
Also, retro computing is in. Vic20, Commodore 64 old game consoles are a definite winner. Atari 2600 taking the lead followed by early Nintendo game consoles. The Generation that got to play the original games (Asteroids, Donkey Kong, and Frogger anyone:-) is now aging rapidly. We would like to play the games we have grown up with. :-)
These are just my initial thoughts, and I am sure you will get a lot of opinions etc. Bottom line, only you know what you can do, you know your tools, your skills and your interests. It will be up to you to find the repair that is in alignment with your values, skills and what you want to get out of it. Money is only a short-term motivator and will always lose to finding a sense of accomplishment. Whatever you do, do it with pride, compassion and purpose.