That reminds me of Project Gorgon, surprisingly still live:
https://store.steampowered.com/app/342940/Project_Gorgon/
they had quite interesting levelling skills mechanics and some social skills, but I downloaded their demo years ago, so I'm not sure where they are now.
I still didn't try Albion online, which should have also some skill progression and something like RP quests?
Generally in MMOs I didn't find almost any RP content, I saw some content even removed in the name of mainstream, so that area is a big unknown.
I saw a lot of players just talk in immersive areas, which was cool, but nothing for singleplayers rp.
I agree that SWTOR is currently the most singleplayer RP MMO experience. And they had that great feature of you being instanced to be able to focus on the story.
I know it is strange to connect that, but I wish MMOs would be about meeting points with other players rather than playing together. I was thinking about it recently. It could be anything RP related, but content, not just players guilds.
Like there is a quest in ESO starting zone, where you are suppose to dress as a servant and they give you an outfit for that. When there are other players around in the same dress, you don't mind that, because you are all servants, but if the story is about you being alone, it can be very distractive to see someone else there.
But they didn't continue with that idea of outfits integrated into the game. Maybe it wouldn't work.