How do you keep your calendars synced?

Right, so I'm guessing I'm not the only one with more than three different client calendars where everyone is fighting to schedule your free time. How do you keep all your calendars up to date? Forwarding invites may share confidential information outside the organization so ideally, sharing just free/busy would be great. I've come across a series of subscription services that want access to your cloud mail hosts' APIs, which - if not sketchy - require your client's M365/Google admins to approve the connection, which may never happen.

At this point, I'm manually creating entries at the time of the meeting invite receipt, but I've just got another client (and calendar) today and I'm dying just a little bit more inside knowing that I have to do it all over again.

Any suggestions?