If you’re looking for a companion app, you might want to try KDE Connect. You can pair the mobile app to your Steam Deck to allow for keyboard/mouse remote input and a bunch of other cool features. There’s also a Decky plugin for KDE Connect in Gamescope.

I suggest Vikunja. There isn’t a mobile app to my knowledge, but the web UI on mobile and desktop is very responsive. It should cover the rest of the items on your list