I use radicale, DAVx5 and Simple Calendar Pro. I tried to use GNOME Calendar on the desktop, but its caldav support is too buggy to be usable, and I ended up using the calendar on my phone only.
Before simple calendar I used Etar, but it had a very annoying bug where it changed the date (or time, I don't remember) every time I edited an event. Plus, simple calendar can send birthday notifications if you have them in your contact book.
Warning:This project is no longer maintained due to lack of interest
Unfortunately here in Italy there's a lot of push back every time some food innovation is on the horizon. GMOs, insects, cultivated meat... In an attempt to save traditional products, we give up on these opportunities without even trying.
We'll see what will be the effect on our economy, but for sure we're losing on the climate side.
I use it to synchronize media to my raspberry pi, which then runs restic daily to do the actual backup.
However, you shouldn't use syncthing if you want to delete the files on your phone after they're backed up, because they'll disappear from the server as well. I think you can wrestle syncthing into handling this correctly, but it's not advised as it's not meant for backups.
For that I've been thinking about setting up immich or the like, but for now I have plenty of space on the phone ;)
You could use mbsync to automate this. You can configure it so that it downloads new mail with IMAP, but keeps them even if they are deleted from the server.
Then you will have a Maildir that you can manage with any mail client, index with notmutch, etc.
Why disallow root login? I always need root when I connect, and stealing the password by aliasing sudo/doas is trivial. It seems to me it would just make life harder for no benefit.