I agree. It sounds very much like an employment relationship - the fact that you can't/don't feel able to choose your own projects is a big red flag here.

If this were a standard situation within Europe/the EU, I'm confident you would be entitled to claim the rights of employment, including e.g. paid holiday. It's possible that this right would extend to a Chinese employer, although I'm not sure.

If you are genuinely self-employed, you should also have the freedom to employ other people to cover your work.