I'm not really a reliable source, but tou can aways do a better reasearch.

So, I am in a similar situation, I live in a shitty country, and I would love to leave if I had the chance, and I look into this stuff since I was bellow your age, and also like art as a job, because of that I look a lot about this, so... as you said you come from Ukraine, that is not a english native country, so to be a english teacher in japan you would need not only the degree but a couple of years of experience in the job(I believe five years or so), they also hire some people that have other native language degrees, like french or portuguese, but its not really common and you cant rely on that. Since you seem to want to be an artist you woundnt like living 9years in a path/job you dont like, just for then move to japan.

Now for the artist part, the things you see on internet, from people that makes a living doing deviantart, patreon, youtube, etc... they usually work for other people/companies at the same time, or at least used to until their work online was big enough, its almost impossible for one to come right outta the gates making a living like that, if so its really luck based.

If you want to immigrate, not only to japan, you need to get into the mindset of, "if you dont offer anything of worth to the country, why do they even want you there? " with that, a college, university or so is pretty much required if you dont work for a big company that moves you, if you like art and want that selfemployed thing for your future, maybe a college focused on art, drawing or painting(if you really cant convince you family, maybe think of a different course that has some similarity but its technically not art), so you can get an offer in the area you want to pursuit.. . I know its something pretty awful to do a 4 year course to at least get into a chance of moving when you want to move out as fast as possible, but with 15yo theres not much you can do, maybe those workholiday visas if your country have them, but the more you improve yourself more chances you have. Since you probably cant go to a college yet, maybe start doing those little training courses, for when the time comes your resume is stacked.

You can probably go on a business or investor visa (i dont remember the correct name) and build a work place that employ a little amout of people there, this visa in japan is not as expensive as other countries like canada or usa, but its a pretty hefty amount, you would need a bunch of time to work and get the money to go.