I've had some success running their windows software via Wine. Haven't tried firmware updates like that though, didn't want to risk bricking it (:

You can look into selfhosting Forgejo! No paywalls, and super fast, especially compared to Gitlab

Localsend is great

For SailfishOS(Jolla) you can check here: https://forum.sailfishos.org/t/banking-apps-on-sailfish-os/18438

Many people equate the word "daemon" with the word "demon", implying some kind of satanic connection between UNIX and the underworld. This is an egregious misunderstanding. "Daemon" is actually a much older form of "demon"; daemons have no particular bias towards good or evil, but rather serve to help define a person's character or personality. The ancient Greeks' concept of a "personal daemon" was similar to the modern concept of a "guardian angel"—eudaemonia is the state of being helped or protected by a kindly spirit. As a rule, UNIX systems seem to be infested with both daemons and demons.

From https://en.wikipedia.org/wiki/Daemon_(computing)

Boombrush is a great choice

No full guarantee, but scanning all downloaded files using a tool such as malwarebytes can help catch a lot of malware.

And ofc the most important one: Common Sense! If something seems too good to be true, it probably is.

TerraBrush is quite nice. Not as complex as other terrain editors, but good and easy to use for basic terrains.

Obsidian + Syncthing has been working prefectly for several years now for me, across Windows, Android and Linux.

The systemd website itself seems quite information-rich: https://systemd.io/

Since she likes puzzles, the Trine game series could be nice! I think they added coop multiplayer starting with Trine 2.

If you both like hack 'n slash then Torchlight 2 is also great fun!