So... this is going to come across as a weird suggestion, given that the company providing it is actually evil, but... I'm actually getting good usage from Google Play Pass at the moment. A £5/month subscription gives access to a wide range of Android games with absolutely no in app purchases and no adverts. I'm not sure if any MMOs are included in the catalogue: there's like a thousand games in the program and I've tried out 4 so far. Because that's the thing I've really noticed. With no in app purchases, I play a game for longer as I don't run up against demands for money, adverts, or difficulty walls that are intended to get you to pay. So I just keep playing until I've done everything I wanted to do.
If you can tolerate the evil that is Google, it might be worth getting the free trial month and see if there are any MMOs.