I do this too. Took me a little effort to set things up, but now its so easy.

They should implement some form of pgp into private posts so only folks with the right key can decrypt

I would greatly appreciate a bit more detail on your setup, is your calibre library simply a folder synced through next cloud?

I had a similar issue with using tail scale, but here the issue was definitely not on the client. My actual DSL connection would reset, multiple times per day. Had the provider come 4 times to investigate the issue, got 3 new routers, they reinstalled the entry point to the house, and fixed an issue in the neighbourhood central point. All to no avail. I ended up purging everything tail scale and hand rolled wireguard. No more issues :/