My go-tos at the moment are:

• Oldschool Runescape - wooooooooo
• Danmaku Death - old, free, no in-app purchases or ads bullet hell game from 2014, but it's delisted from the play store...

Other games I've enjoyed

• Super Auto Pets. Base game is free with no ads, but there are cosmetics and expansion packs.
• Pixel Dungeon and its variants. Free roguelike with no ads and no in app purchases.
• Magikarp Jump. Fun little incremental game where you make magikarp jump. Has in-app purchases.

It's as good as it used to be. I've enjoyed doing quests and fighting bosses. Grinding is slow but tolerable if you have a video or podcast you wanted to watch or listen to. Coming back as an adult is a different experience. You'll find yourself being a bit more directed in how you play and it's easy to optimise the fun out of the game. It's decent on mobile and I have heard of people fighting endgame bosses on their phones.

It is easy to get overwhelmed on p2p. My advice is you set a goal and go for that, or you pick a thing you really want to do and just do that and let the game slowly open up from there.