I've used twitter as well and sometimes plug a little on my comics here on tapas.
But referring to Twitter I'm actually not to sure what specific tags do best but i try to keep it fairly simple with #commissionsopen or #commissions and I'm starting to try using my own unique tags (#vibrantfoxcommissions) though level of effectiveness has yet to be determined. Sometimes I've seen tags specific to the commission type like chibi, digital, traditional and so on.
It's also helped me by keeping my commissions tweet pinned to the top of my profile though I know some folks who include it in their general introduction threads but like anything varies from person to person or profile to profile.
I've also advertised or at leas had commission info posted on instagram though I havent really gotten any hits there. There's one post with the commission info, then I share the sheets/prices and samples in an ig story, then add those stories to a highlight made for commission info (hopefully that makes sense)
I don't really know many other sites aside from the ones mentioned and the only other that comes to mind is artists and clients though i have no experience on it myself.
https://artistsnclients.com/
There may be a few testimonials or something around but other than that my knowledge on the site is limited at best.
Ko-fi is kind of an option but you can't use their official commissions feature unless you've upgraded to gold
