While not strictly aimed at composition but rather music production in general, the top channel in my book as a REAPER user is REAPER Mania by Kenny Gioia. This is (IMO) a rare example of sharing invaluable skills by a true expert in a simple, no-nonsense way.
Speaking of composition, I do follow several channels on the subject, especially Guy Michelmore who often makes me smile in the process. I have also found many good bits of advice on Spitfire's channel, being a user of a number of their products. However, one of the most inspiring and enlightening channels I've come across yet is Project SAM's official channel. Those short videos on composing in different styles are pure gold. I wish there would be more educational videos with this kind of approach out there.