Thank you all for suggestions! I really appreciate them 
@Takiro - that’s a really good advice about the books. I searched through some bookstores if I can get some of the highly rated books in my language and it looks like I could order them.
I have the opposite problem with youtube videos - they are often far too basic and they don’t really show to practice it, just tell the knowledge, but I think I have the knowledge - at least at that level, and in some cases, often on higher level, for example how light and colors work and how to approximate them on the painting, it’s just, I’m still bad at colors in painting itself… And perspective videos are all talking about very basic things that are quite easy for me (I’m, like, really good at math and stuff which I feel helps) while most of my problems with it comes from judging the distance and depth (alternatively, laziness, when I don’t want to use rulers and just assume that something is good enough
) and most videos don’t show any tips on that. I’ve seen a tutorial/book on how to draw the same length in different distances from the viewer, that was helpful, but it was also the only place I’ve seen this advice (it seemed correct, just “advanced”).
Warming up definitely seems useful and collaborating might be a lot of fun, too, I’ll think about it. And the fun part - yes, that too, otherwise the passion might easily burn out…
@ArtAvenue Generally I agree, that’s a good tip overall. But I learned like that for a long time already, I feel like the knowledge is a bit scattered now, I feel like I’d want some external guidance.
And I agree with the lineart tip, especially learning how to use the first tablet is much easier when one does painterly paintings than lineart… even a sketch can be very frustrating to make.
@FloDX Yeah, I feel like I’ll need to check Proko again… I did check him out earlier but it was different back then.
Also I believe I wasn’t really clear on the art school stuff… It might be a cultural/country difference, too. So let me explain how it works in my country and how I wanted to use it.
Generally there are art colleges, the most prestige ones are called “pretty art academy”, yes, seriously (I guess the correct translation would be “Academy of Fine Arts” or something). The prestige ones are usually, with some rare exceptions, public schools, which means you only pay if you cannot pass to the next class or if you are on special evening course etc. It also means that there is a huge competition to get there. There are art exams and you need to show the portfolio. It’s also true if you want to go to architecture course. They basically expect you to know how to draw before going to art school or the architecture course, and the exam is quite difficult since you need to be better than all others.
To get to that art academy, you can first go to an art high school. But if you haven’t, and there are some reasons to not go, and it might be that you decide to go to art school much later than when you decide on a high school, then you need to learn it elsewhere, so there are those… art courses, art workshops, that prepares you to the exam. It’s an extra-curriculum, like activities you do after your normal school. They know how to make a good portfolio and what can be on the exam, and they can prepare you from 0 to someone passing the exam in max two years. They’re extremely effective (usually one year is enough to get very high level). Of course if you want to go to art school and that’s why you go to that course, you need to work hard, but I’ve seen results. So I was thinking this kind of school - focused entirely on art skills, no art history or anything else. They usually have hobbyist courses, with a bit less pressure and less time in a week (cause the academy-preparing ones would be at least twice a week etc., too).
Ahh and also, for reference I wanted to show some examples of my art so you know what level of art we’re talking about.
Lineart/sketch from reference:
Lineart/sketch without reference:
Full color, from reference:
https://www.deviantart.com/tiarevlyn/art/John-Watson-759714923
Full color, me coloring someone else’s (Jazza’s (from Draw with Jazza), more precisely) lineart (no reference):
https://www.deviantart.com/tiarevlyn/art/Post-apo-grief-727191014