Gaming

I use mine for pac-man, tetris, and space invaders. Anbernic. I played a little GoldenEye too

Yes! I just got my Retroid 4 Pro, and I absolutely love it. Here it is running RetroArch's mGBA emulator, with Pokemon Dreams.

I've mostly been using it for emulating, and it really really excels at that, far more than my SteamDeck does, honestly.

I don't know that particular system, so I can't give any hardware-specific suggestions. However, it might be worth checking out the games that previously were included in Humble Mobile Bundles, especially the very first ones. Sorry, I don't have a nice to browse list, but this site seems usable enough: https://barter.vg/bundles/3/76/

That said, OpenTTD is available on F-Droid. That should keep you busy for a while 😉

Endless Sky is a cool space trading game if that’s your cup of tea.

No experience with the system; I just stuck a controller cradle to my phone instead and am using a Steam Deck for more serious mobile gaming, but there are a few obvious games that should work with it:

• Pixel Dungeon and its many variations: Incredibly hard, seemingly unfair roguelike, but there's a system to the madness and you can figure its very systemic gameplay out with lots of patience. Shattered Pixel Dungeon is the most popular variant.
• Mindustry: Popular open source clone of Factorio. I think you'll need to fiddle with it to get the controller portion of your device to work with it (your device might have a built-in tool for this).
• Minetest: Open source Minecraft clone and a pretty decent one.

To be perfectly honest, the open source gaming scene is pretty small and among the few games there are, even fewer are both available for Android and make sense on your device.

You should however look into source ports, in addition to emulation, of which there is a bewildering variety (disclaimer: I have not checked if any of these require more processing power than your device can provide):

• ZDoom: Should be obvious what this is for.
• idTech4A++: Allows you to play DOOM III, Quake 4, Prey (2006) and, I think, a bunch of mods on Android.
• Xash3D: For Half-Life and its many, many mods.
• OpenMW: Open source engine for Morrowind. May come with the risk of never putting the device down ever again.
• OpenLara: As the name implies, this one allows you to play Tomb Raider 1-4. Comes with lots of neat enhancements, although it obviously doesn't go as far as the new remaster of the trilogy for PC and consoles.
• BuildGDX: For Build Engine games like Duke Nukem 3D and Shadow Warrior.
• JoiPlay: For RPGMaker games, among other things.

More of these: https://emulation.gametechwiki.com/index.php/Game_engine_recreations_and_source_ports

Keep in mind that only those with an Android icon are for Android and that not all of them are suitable for a device with a screen this small (SCUMMVM for old Lucas Arts adventure games doesn't make a lot of sense, for example).