I would ssh into the opnsense box and press 8 to run the shell terminal and then run dmesg and go back to the time the server rebooted, there you can see the events leading up to the shutdown.
I set up flexo for Arch Linux update caching and squid proxy for Alpine, Debian. This stops me from having to download the same files over and over.
Hey I do this exact thing as well. Would highly recommend if you like self hosting podcasts to listen to.
I use zabbix to monitor everything, agent on each device uses around 30 mb of memory and with the Linux templates it can monitor just about everything on the server.
The way I get around the syncing issue is to set my syncthing to sync when my phone is charging so it’s very unlikely to not be in sync, or if I change a password on the PC I’ll plug my phone into a USB and it syncs straight away.
I also use KeepassDX on Android and never have those issues.