What also works is using SyncThing. That way you can sync any app that uses files that you want. I use Obsidian for note taking which creates .MD files in a designated folder, I sync that folder with my phone.

For those with a robot vacuum you might want to look into Valetudo. Or just not connect it to the internet.

Balatro, Slay The Spire en Hades 2

Can’t you just use Thunderbird as desktop app?

Mailbox.org (option if you want custom domain) or Posteo.de