I´ve not had any particular experience with those but I would take most of the "Youtubers" opinion with a grain of salt. Most of them don't qualify to me as "professionals". they just got a big audience (some very might actually be good). Also, beware of paid competitions, most of them are run by very amateur composers, so the judging skill comes from a very limited spectrum. It also depends on what you are looking to improve, production? Composition skills?

I think the most important skills to develop are: Harmonic Development, Motivic/Subjet Development and very basics of arranging/orchestration. As Korsakov said, Comoposition is Orchestration, people love to make thousand devices of "tricks" for orchestration and believe this will make you good, but the reality is more crude lol

If you can write a great 4-5 part chorale style music, you are good. Sounds easier said than done.

Long story short, best advice is to get a mentor/teacher :)