Selfhosted Calendar Server that works with the G?

So, I’ve got a homelab thing going, with NextCloud and Immich and other things… using Docker. But the one thing I can’t seem to wrap my head around is how to get my calendar needs met behind the scenes. I’m hoping there’s some sort of calendar syncing server I can use.

The problem: I use NextCloud and another android app to use my calendar. Easy. Historically, my household has used a single Google Calendar shared amongst members.

The real problem: They won’t ditch Google. But I need a solution so that I can:

1. Sync their calendar events to my NextCloud calendar.
2. Sync my calendar events to their Google Calendar. I think that the issue here is, the share links that the calendars give, can only be used to list a new separate calendar on each of the respective platforms - not sync the events between the existing calendars…

I’m hoping that there’s maybe a syncing solution out there that can play middle man and sync the two calendars directly, rather than having separate calendars for the two environments?