Considering what my ISP is I think I’d trust any vpn with a FOSS client over them…

I use mailbox.org because both proton and tuta do not support IMAP, which is generally needed to use a 3rd party client. Mailbox.org is also pretty cheap and is pretty private as far as the non-E2EE providers go. And in terms of E2EE it is going to be used so rarely that is basically doesn’t matter, the type of person that would use tuta/proton would probably be willing to setup PGP with you, and E-mail isn’t a secure communications platform anyways, so you might as well just use Signal, XMPP, briar, etc…

The only justifications for using flatpak are having a musl system or if the software has way too many dependencies. Use your package manager when possible, otherwise use an appimage, or find a way to get the .deb or .rpm working on your distro.