Accept/Decline (RSVP) by external attendee via email (Google, Microsoft, etc.) not working

[pat-githubber]

Is your feature request related to a problem? Please describe.
When creating an event with the Nextcloud Calendar and inviting an external attendee with external email address (e.g. google), the RSVP action in the invitation email (accept/decline/maybe) executed by the external attendee does not update the event in the Nextcloud Calendar.

I researched this problem and found many long discussed issues, which are closed but the problem still remains.
Note: I am a simple user - no Github experience. I do not understand some of the discussions.

#2338
#19144
#2149
#2338
#9942
Nextcloud Help
Nextcloud Help

Version
Nextcloud Hub 8 (29.0.5)
Calendar 4.7.16

The problem in detail

1. I am the admin on my Nextcloud server incl. having a user profile with email address

2. I have a calendar on my Nextcloud server

3. I create an event and invite an external attendee with external email address (e.g. google)
   

4. I save the event.

5. The external attendee receives an invite
   

6. The external attendee accepts the invite with “Yes” using the regular RSVP provided by google (like basically every professional email user neglecting the “Accept” and “Decline” buttons depicted by Nextcloud in the email)
   

7. Again, the external attendee (will always) neglect “Accept” and “Decline” buttons provided by Nextcloud (and use the RSVP feature by google, Outlook, etc.)
   

8. I receive a confirmation email that the attendee has confirmed the invite
   

9. However, the event in Nextcloud calendar remains as “Has not responded…”.

Describe the solution you'd like
When step 6. is executed (e.g. accept), the event in the Nextcloud calendar is automatically updated accordingly (e.g. accept)

Describe alternatives you've considered
Not using Nextcloud Calendar and Email - only Files.

Context
I really would like to make Nextcloud use for my organizaiton. However, I cannot imagine how any company can use Nextcloud Calendar i.e. Nextcloud as a whole without this feature. Then I rather use e.g. Proton Mail and Calendar and solely Nextcloud Files.

Not being able to accept invites on iOS is also a deal breaker.