Migrate Google calendars to iCloud?

I’ve had two Google calendars for decades and use Apple Calendar for MacBook, iPhone, and iPad exclusively. Our family shares our calendars between, and generally had no issues. We all have the same setup (I’m the family CIO)Apple’s calendar app on iPhone, Family Sharing, all calendars turned off except the google account. . Recently, I found out that two members can’t see the shared calendars on their iPhones. In trying to fix it, I’ve screwed it up for everyone but me, even on Google website. I give up!

I want to switch entirely to our iCloud accounts for iCal to fix the sharing issue but I need to migrate our calendars to iCloud. How do I do that? We have many past and future events, repeating events, notes in events, etc. that we need.