How do I keep iCloud from clobbering my data?

Everytime I activate iCloud on my old macPro (OS 10.11.6) it erases all my local Calendar entries and I have to spend the day restoring from Time Machine. I have never put my Keychain data into the cloud or upgraded my Notes for fear of losing critical data in the process and of losing control of that data. Unfortunately it looks like there is no way to migrate certain Keychains to a new machine (OS 11.4) without using the iCloud service.

I figured there would be a way to clear the already uploaded iCloud Calendars so there would be nothing on the cloud side to replace the local data as it syncs, but cannot find a way to do that.

How do you activate iCloud and migrate critical data (specifically local keychains and calendars) to a new machine in a way that assures the data on the original machine is in no way threatened or lost in the process?

This is a little too obvious, but...

Export the local data first, then if necessary, import again after connecting to iCloud.