ACrockz So can it be that DAVx5 can solve my first issue too to sync Gcalendar with GOS to be presented and maintainee by acalendar?
Not directly (at least AFAIK). You will still need a caldav server for DAVx5 to connect to. The caldav server could import & sync the Google calendar, then DAVx5 would sync from there to the phone.
I've used Nextcloud this way in the past, but that is probably overkill just for a calendar. I'm sure there are other options.
ACrockz Does GOS contain Android calendar function?
This is the needed basis for acalendar to work with...
Yes. GrapheneOS has the AOSP calendar backend, just no client app to display it.