Replacing Google calendar has been one of the hardest apps/services to replace.
We use a shared Google calendar at home and I couldn't finding an app & service combo that is affordable.
I looked at Fastmail to replace email & calendar, but it's very expensive for a family of 6. I wish they did a family plan.
Solution
I opted for a WebDAV shared calendar using NextCloud on a Digital Ocean droplet. I then installed the DavX app to synchronise with the calendar app on my phone.
I now need to replicate this setup on the other family members' phones.
But I still have to use the Google Calendar app on my phone because DavX doesn't sync with other calendar apps it seems, at least until I replace the Android OS.