Realistically speaking, find a tutor. I didn't learn music from primary school either. I basically learned it from a teacher at first. Finding a teacher alone is definitely more expensive than studying in a small class, but it is necessary to be professional. If the economic conditions are not very good, find a professional student to teach you. Now students are eager to work, so are less famous teachers. Nine times out of ten, there is a charge for coming to the door, and the water is better than the outside class. To be honest, I've always wanted to recruit more students to teach music theory or compose music or something ~ ~
PS~ I can try to help you contact some high-level friends if necessary ~ ~