Pit people can be decently difficult on base difficulty and really hard on insane. The missions are generally pretty short. But I think you have to finish a mission, not sure if saves can happen in one. Battle Brothers is another good one, but saving in a battle not sure. Usually those missions are 10-20 mins I'd think but insane end game ones after like 100 hrs playing could be like an hour. The game is also very challenging. There's also tactical breach wizards (new to that game so dont know all the specifics). Fell Seal is another. There's all the classics if you emulate like final fantasy tactics (recommend the GBA version) and tactics ogre (recommend the PSP version) that these are basically all based on. One game I'm super hyped for soon is metal slug tactics. The demo was challenging, really fun and works a lot like into the breach/FTL as far as the overworld. There's also wargroove thats more like advance wars. Hope this helps! Tactics games are generally my favorite genre.

Shorter sessions.
First comes to mind is the recent Tactical Breach Wizards great story good character synergy gameplay.

On mobile I used to play the heck out of Crowntakers (it's on Steam - no mobile monetisation bulldust) but it's more classic hexagon medieval fantasy party, but it had a rogue-lite and the game progresses with small levels. Characters unlock abilities through achievements and it had a lot of replayability.

Invisible Inc. if your looking for something different. It's stealth based rogue-lite

XCOM Chimera was a shorter experience, but it did get mixed reception. It goes on sale often for cheap.

Been meaning to play Wargroove... its on my backlog list to play.

Oh ya invisible inc was the other one I was trying to remember! Highly recommend it. You don't have to worry about rng with attacks like xcom so I really liked it.