olof
Just a quick thought. These days you can create a special partition in the first part of your drive and just boot from it. No grub, no config really. You might want to take a look at EFI and an EFI system partition perhaps?
One idea is perhaps to have more than one device? A tablet at home could be used in some scenarios, the rooted phone for some others. Maybe you have an old device laying around?
Does this run on x86/64 bit systems? Or this is for Linux smartphones more.
Just in case - there are also good linux desktop applications - check out Authenticator on flathub for example.
Jerboa. It ticks all the boxes, is updated, and looks good
My vote goes to Mullvad. Possibly Proton VPN as a second contender.
Please dont link with a Google Amp link.