I also use Nextcloud as the backend and iOS Calendar app as frontend on mobile and Gnome Calendar as frontend on PC.

It mostly works great, except for 3rd party Office365 calendars imported into Nextcloud. It has issues with some events on some clients displaying correct time when event invitees are from different time zone.

I've been using Nextcloud as well for about 5 years now, and using Etar on my android. It's been great.

Nextcloud calendar is a bit annoying in my case, as email reminders often come late, sometimes 10 minutes or more. I guess it's because they rely on the cron process.