Same here. Have you found any viable alternative to the Google Calendar? I'm at the point where I'm thinking about hosting a calendar project from GitHub myself.
Nextcloud, whether self-hosted or otherwise, works great! It's just WebDAV. You can get calendar, contacts, task, and note syncing, and it can even host your documents for reference management software like Zotero.
The calendar of Fastmail works.
It's not great, but it does the job
If you own a Samsung phone, the calendar app is good. I wonder if you can install those Samsung apps (which for some are just forks of unmaintained AOSP apps) on a regular Android if you somehow get the apk.