I have no opinion about rsync.net. I’d check which services restic supports; there are several, and it is it supports rsync.net and that’s what you want to use, you’re golden. Or, use another backup tool that has encryption-by-default and does support rsync.net - there are a couple of options.

I would just never store any data that wasn’t meant for public consumption unencrypted on someone else’s servers. I make an exception for my VPS, but that’s only because I’m more paranoid about exposing my LAN that putting my email on a VPS.

restic, and other backup tools, are generally not always on. You run them; they back up. If you run them only one a month, that’s how often they run. The remote mounting is just a nice feature when you want to grab a single file from one of the backups.

What you’re describing is a classic backup use-case. I’m recommending the easiest, cheapest, most reliable offsite solution I’ve used. restic has been around for years, and has a lot of users and a lot of eyeballs look at it, and it’s OSS. There are even GUIs for it, if you’re not comfortable with the CLI. B2 is generally well-regarded, is fairly easy to figure out, and has also been around for ages. Together, they make a solid combo. I also backup with restic to a local disk and use that for accessing history - B2 is just, as you say, in case of a fire, or theft, I suppose.