I also recommend Nitrokey. I have a Nitrokey Pro 2 and a Nitrokey 3 NFC and they both work well. Linux support is very good, and they also have good documentation on how to do most stuff you might want to do. +1 for being open-source as well.

Internet access? Never. LAN access though is a different story altogether…

I use NixOS with single-partition btrfs on both my laptop and my desktop and have never had this happen. Curious…

I’ve been on NixOS for about a week now and I can say I’ve got access to pretty much all of the packages I was using on Arch just from nixpkgs. I even found it quite easy to package stuff myself!