My current 'while I poop' mobile game has been Final Fantasy Record Keeper for quite some time now.
It's a game where you play through all of the boss fights from every main series Final Fantasy game (and Tactics) using a party built from every character, playable or not, from all of those games.
It manages to capture the feeling of Final Fantasy party management and RPG boss strategy without having to advance through story text and dungeon crawls that you've done a thousand times already.
The main mode is a giant flowchart of all of the dungeons from all of the FF games that you play through in sequence. There are trash enemies before each boss in these dungeons, just to make it more of an endurance run and recapture the feel of these old games than just being boss fight after boss fight. There are so many of these dungeons available that I still haven't caught up after a year and a half, and they add more every month.
They also have a new event every week, where they feature one particular game, and a specific character or plotline within it. This is where you quickly unlock new characters from that game and challenge hardmode versions of the bosses you may have seen in the main mode for the best rewards.
Being Final Fantasy, there is an element of leveling up characters and abilities, but it feels less like a grind and more of a ramp-up early on, and the game gives you ways to circumvent leveling up characters manually after you've built up a solid team. It doesn't take long to get to the challenge fights, and it feels like you make further progress with every event.
Each dungeon that you complete in either mode provides some in-game currency you can use to buy a random selection of new weapons and armor - the weekly events contain new items for the featured game every week. Every featured item you get allows one specific character to learn a new unique ability that works like a Limit Break, so you eventually can build a party based on which of your characters have drawn the best abilities, or you can swap them out based on boss weaknesses and strategies. (You can also use real money to draw more items, but this is 100% unnecessary and you can completely clear all the content in the game without spending money if you build a good strategy for each boss. All that spending money does is allow you to draw more of the same items you'd get for free faster.)
So yes, it's a free-to-play mobile game, but it doesn't feel like one, and it really recaptures the nostalgia of playing an early Final Fantasy game, using some combination of characters, classes, and abilities to go up against every boss from every game. I've been having a blast with it for a very long time.