Ragnarok Online was pretty great and you can still play it with private servers. This is a very chill game, mostly about the vibes and hanging out, you'll probably need a friend to enjoy this with because IDK if you'll get much out of it just grinding with random people. When you pick a private server you should probably find one with low exp rates, like 10x or less, if you've never played before.
Phantasy Star Online isn't really an MMO (it's pretty much a clone of Diablo except 3rd person and sci-fi) but it was heavily marketed as one. Grab the "Blue Burst" version for PC and play on a private server. You'll probably want to use a controller for it. This one you could play with randoms.