Does this run on x86/64 bit systems? Or this is for Linux smartphones more.

I would also say this. Njalla is good

Tailscale maybe? They have a mode where you can configure a site to site links, you could route the docker networks. https://tailscale.com/kb/1019/subnets

Jerboa. It ticks all the boxes, is updated, and looks good

Tbh, that’s the only platform where it is needed?

+1 for Organic Maps. Create an open streetmap account and contribute - its a game changer for me. Instead of a consumer, you’re a contributor. (That goes for all OSM ofc)

Yes. I use QKSMS for text messages now instead.