I tried this but using #NextPush instead of NTFY.

Tapping on #UnifiedPush in the #Dav5X settings instantly picked up UnifiedPush and subscribed, showing my account in brackets. The NextPush Android app showed Davx5 as subscribed.

It seems I had to turn the collection off and on a couple of times before it showed as subscribed, and it still takes about 30 seconds to update. Could that just be #GrapheneOS's default calendar app that I'm using?