Im currently playing Shattered Pixel Dungeon, one of many Pixel Dungeon forks. Its not on F-Droid, but it has builds on github https://github.com/00-Evan/shattered-pixel-dungeon

Unciv, HyperRogue. HyperRogue can be addictive but difficult if you stay in one region.

Unciv is an open source 4X civilization game.

Fdroid has a bunch

Luanti is a pretty good start as it has a lot of content

deep gameplay, challenge, real progression (any genre welcome — strategy, roguelike, RPG, racing, puzzle…)

That's a challenge for open source games in general even more so on Android. I can only think of Mindustry which was mentioned before.

Maybe consider to set up a Linux container, similar to what DeckDroid offers for SteamOS.

In the order they appear in my Droid-ify, not in the order of fun or depth, etc.:

https://f-droid.org/packages/com.unciv.app/

https://f-droid.org/packages/com.resurrection.blowtorch2/

https://f-droid.org/packages/me.lecaro.breakout/

https://f-droid.org/packages/net.minetest.minetest/

https://f-droid.org/packages/org.broguece.game/

https://f-droid.org/packages/com.dozingcatsoftware.bouncy/

https://f-droid.org/packages/org.develz.crawl/

https://github.com/TrashboxBobylev/Summoning-Reincarnated

https://github.com/TrashboxBobylev/Rat-King-Adventure

I've played and keep playing all these games, some more than others. The last two are not on fdroid yet, but they are foss and amazing.

BlowTorch is for mudding, and mudding is deep, mature, highly active, and free.

P.S.: Oooooopssss!! Forgot one of my all time faves, goddamnit. Here it is:

https://github.com/cataclysmbnteam/Cataclysm-BN

You won't find a long list. For one, check Mindustry.