Shattered Pixel Dungeon

Unciv and mindustry are two of my favorites. https://github.com/yairm210/Unciv https://github.com/Anuken/Mindustry

Chess on Lichess ticks all your boxes except maybe "modern" (though it has a modern UI).

I like to play chess puzzles on the shitter at work, while traveling or at home when I'm bored with other games.

Endless Sky is available from F-Droid. I personally found the UI to be a tad clunky, but that might simply be because I'm used to playing it on desktop. You might find that worth your time.

Idle Fantasy

I like Breakout 71 by Renan LE CARO

Stardew Valley isn't open source but has the rest of it

D2Android

It's not quite what you describe but I'd argue it's very much what you're looking for. It runs Diablo 2: Lord of Destruction on Android with some quality of life mods and the option to mod it more heavily.

D2Lod is actually very different from D2 Resurrected or Eternal, mostly because it was made when games weren't live services. There were a lot of changed made to D2R that increased the grind which aren't present in D2Lod. It's the full game with an engaging story, cinematics, decent replayability, a solid gameplay loop, and you'll probably recognize the birth of a lot of familiar mechanics in it (borderlands is literally just d2 with guns)

It's a really reliable and open source app, I've been playing for weeks and it only crashed once on the very first run of the game, but I think that's because it took too long and graphene killed it. It does use the original d2Lod files which aren't open source but they were programmed for windows 95 so they're closed source like a picket fense is secure.

That is possibly the only issue I expect you'll have with the game, it's by no means retro but the graphics fit the age. Great for what they are historically but even I can see the improvements made for D2R. Lastly you do have to own a copy of D2Lod and use the cd keys to activate it. The github tells you how to get them officially but I'd be happy to just give you the files and keys from my 2001 Battle box. I bought the game and I'll gladly remind Blizzard of that every chance I get.

pokemmo

mtg forge

I would really love to see an Open-Source clone of Plants Vs Zombies.

(No I cannot code)

That Twilight Princess port called Dusk lol

Battle for Wesnoth has an android version. Works quite well.

Andor'd trail. I sank so many hours in it that I had to uninstall lest I become negligent of my daily tasks and work. Also has some funny moments.

I'm sorry I don't have a list to add to the conversation but one of your points made me curious:

What do you mean by "Made for adults: deep gameplay, challenge, real progression (any genre welcome — strategy, roguelike, RPG, racing, puzzle…)"?

As far as I'm concerned, real adult "games" don't have progression: Basketball, football, baseball (do you count muscle growth?), chess (ELO is progression? one value?), backgammon (ELO I guess?).

What do you see as "real (adult) progression"?

edited (to make my misunderstanding bold)

Chip defense, a casual tower defense game that teaches you about circuits and is surprisingly more in depth than it deserves to be. https://f-droid.org/packages/de.chadenas.cpudefense

"I keep having this dream" is pretty good, I thought

Balatro.

One of the best games to play on mobile. It fits basically all of your criteria and you can just Google search a copy of it, you don't have to pay for it from the play store.