Hmm… ProtonVPN team solved this in better way. They put the repo configuration stuff into DEB file, so it’s just a matter of double clicking it and clicking install

I was wondering how they'd solve signature checking and key installation - and looking at their page they seem to recommend skipping checking package signatures which, to be honest, isn't a super good practice - especially if you're installing privacy software.

Please don’t try to check the GPG signature of this release package (dpkg-sig –verify). Our internal release process is split into several part and the release package is signed with a GPG key, and the repo is signed with another GPG key. So the keys don’t match.

I get it's more userfriendly - and they provide checksums, so not a huge deal, especially since these are not official Debian packages, but the package signing has been around since 2000, so it's pretty well established procedure at this point.