Have you tried the .ics download method?

I thought that the OS would see the ics file and automatically know to open your calendar app to deal with it.

The big issue is it breaks the rule of not opening files or clicking on links. So for people like me, state the event in an email so I can do my own entry.

Other then that I would think you could just provide an ics file either attached or as a downloadable link.

For link I think any site that allows you to publish files by link would work. Pretty much any box site like Proton Drive, Trezerit, Nextcloud, Google Drive, etc. Also send services like Bitwarden Send or derivatives of the old Firefox send.

Framasoft has this: https://mobilizon.fr/

It sounds like a Radicale server set through an ngnix proxy may be a solution for your needs?

Other than the last part for you, but once it's set up it seems like it's pretty much going to run forever edit: and is easy for your users who get the invite url.