You should be able to use CalDav to sync your Google Calendar into a different calendar client. Of course, this still means you will be using Google Calendar, but from the comments, it seems you want to stick to it for the time being.
I have a GrapheneOS device (and do not have a Google account in my personal life), however my work uses the Google suite of products, including the calendar. So, I have DAVx⁵ (https://apt.izzysoft.de/fdroid/index/apk/at.bitfire.davdroid) CalDav set up along with Etar (https://f-droid.org/packages/ws.xsoh.etar/) for UI and widget. This works well for me without relying on the official Google Calendar app and Google Play Services.