Disable iCloud keychain on one device

I have a problem with the iCloud keychain. It seems to centre around my upgrade to Big Sur. I cannot do all of my machines so one remains on Catalina. Shortly after the upgrade I find I'm getting authentication failures on all of my accounts - fast mail, google, and office 365. Entering the correct password does not fix this and it just says verification failed. The only solution that seems to work is to delete the account from Internet Accounts and then put it back. Unfortunately, that fixes only one of the two systems. If I fix it on the system running Catalina, shortly thereafter the machine running Big Sur has problems. If I fix it on the machine running Big Sur, the Catalina machine has problems. I've been attempting to disable the iCloud keychain but it refuses to turn off. While it's not my preferred solution I would much prefer to configure each machine/iPhone/iPad separately and have everything work than have an upgrade/fix of one break the other. I assume this is some kind of conflict between how Big Sur handles keychain objects vs Catalina but upgrading the system on Catalina is not an option at present. I've done a number of searches and tried numerous things listed in this thread: https://discussions.apple.com/thread/5472218

Nothing seems to work. How can I effectively kill the iCloud synchronization of my keychain?