Logseq can be launched as a desktop app and in a web browser and has both iOS and Android apps. Official sync is a paid feature but there are other options (e.g. Syncthing).

Applying negative selection, QNAP has a terrible security track record and WD has terrible performance and security track record. So if you want something that's mostly plug and play, Synology it is, consider a "+" model if there's any chance that you may expand beyond the simple file sharing use case.

Disclaimer: I currently own three Synology boxes (718+, 220+, 216j) and a QNAP TS-453D running Proxmox (for fun and as an attempt to move towards a HW vendor-independent solution).

Navidrome with Substreamer on mobile for auto-generated recommendations based on already played music works great for me.

For a less obvious use case, I did some light video editing with Kdenlive and an external monitor, mostly because the Steam Deck is more powerful than my laptop ;)

Also PS2 emulation, Burnout Revenge has not aged one bit and plays very conveniently on a handheld.

Jitsi also provides integration with Matrix chat.

Keeping storage and compute separately is the best practice but if you're OK with combining both in one device then running docker-compose on a Synology (via SSH, + versions only) works just fine. An alternative that reduces the lock-in at the expense of more tinkering is buying an amd64 QNAP and installing Openmediavault / TrueNAS / plain Debian.

Substreamer on iOS, I've been getting surprising amounts of mileage out of the "play random song from your library" feature recently.

Heroic for Epic, Lutris for GoG, this approach worked best for me so far, I ran into issues when trying to run both stores in one application.

Calibre Web has Kobo integration, although I'll admit I haven't been able to make it work so far: https://github.com/janeczku/calibre-web/wiki/Kobo-Integration