Just wanted to leave a comment to say thank you for sharing your findings and taking the time to write your post here. I am sure you spent a lot of time and effort researching all of this. These kinds of posts are why I love Lemmy and its communities.
Not currently in need of a new VPN, pretty happy with Proton and my plan is active for another 1.5 years. But I still wanted to say I appreciate your post.


XMPP knows invite links, there are even servers which support invite pages which list suitable clients one could use for XMPP. Prosody for example has this module you could enable: mod_invites_page
And then there is also Snikket, which is a pre-configured XMPP server suite (running prosody under the hood) with some nice QoL improvements baked in which you can selfhost as well: Snikket Quick Start Selfhost