Pretty much every YouTube musician/composer I saw who made the switch to making music their full-time career was either a video game music composer (e.g. Waterflame) or used Patreon to get that full-time career-level money stream (e.g. Jonny Atma of GaMetal fame). For the former case, I don't know how much uploading to sites like Soundcloud or Bandcamp would help you get your foot in the door, so to speak. You probably still have to apply for job applications for video game music composers or, at the very least, get your music in an online game or several for free (e.g. participating in hackathons). You will probably never get enough money from YouTube revenue alone to quit your day job.
Sheet Music Plus does host SMP Press, which lets you sell your sheet music through them, but I have no clue whether anyone who sells through them got to make music their full-time career.
Regardless, you gotta market hard (to the point that starting your part-time music career earlier likely gives you an edge). Releasing your music to multiple websites is probably a good idea. Make sure to actually fill in the descriptions of your YouTube videos (this is one of the most common places to find the musician's website, Patreon link, music file download link, tabs, the Bandcamp link of the song the video has, etc. from my experience). Making a Facebook and a Twitter account probably also helps, but make sure to keep updating your feed in both of them or your fans will stop visiting whichever website you haven't made updates in.
You get additional cred if you're also a member of a band, even a former member. (I've seen musicians in bands, such as Christian Muenzner, also have solo Bandcamp accounts, and one of my former project managers is promoted on the Bandcamp page of the (probably little-known) metal band "Every Hour Kills" as "ex-Divinity".)
The heavy implication I'm getting is that, if you publish music solely online and don't play physically in bands or buildings, you'll have to be prepared for a part-time music career for years, if not the rest of your life. As far as I can tell, anecdotes like LittleVMills/Little V's "I'd really like to move out of my grandparents basement (yes you read that right). Gigs are drying up locally and my only real stable form of income is teaching which barely pays the rent and bills." (found on his Patreon page) or worse are commonplace.