Is there any way for someone to share with me a calendar via iCloud and I can see it but NOT have any of my own calendar data on iCloud?
It is not a public calendar that I can subscribe to. It’s one shared via an iCloud family sharing plan with a different Apple ID than my normal one.
The way I read this you do have to have iCloud turned on but you should be able to turn off mail and other services individually. As long as your own calendars are “On my Mac” it should work, IMO.
As soon as you turn on iCloud things will start to sync so be prepared to start turning off individual services. In fact you might want to disconnect from the internet immediately after turning on iCloud.