If that’s all you’re after (Contact, Calender, Drive) you may well be able to just plug a hard drive into your OpenWRT router (it has https://openwrt.org/packages/pkgdata/radicale2 which does caldav and carddav), work out sharing (apparently at least samba works) work out how to back up the drive (plug in two, mirror and unplug one, RAID is not a backup) and call it a day. I don’t have one, but it seems likely doable… Tailscale in when you’re out and about…
If that’s all you’re after (Contact, Calender, Drive) you may well be able to just plug a hard drive into your OpenWRT router (it has https://openwrt.org/packages/pkgdata/radicale2 which does caldav and carddav), work out sharing (apparently at least samba works) work out how to back up the drive (plug in two, mirror and unplug one, RAID is not a backup) and call it a day. I don’t have one, but it seems likely doable… Tailscale in when you’re out and about…