I need to disengage from iCloud

Here’s a good write up! How to set up and access iCloud Drive on the Mac

But before you break the connection you’ll want to make a copy of whats on your iCloud drive presently. Go to the dock and locate the iCloud Drive icon to open the finder in that view drag copy it to your local backup drive. You might want to do a few folders at a time so you don’t lock up your connection in case you need to do something else. It can take a bit of time depending on your connection.

Once done make a Time Machine backup or use what ever backup application you feel comfortable with. Do take the time to check it is really backing up and you can recover your data either a full restore or selectively.

You want to delete your iCould data follow this Apple T/N Manage your iCloud storage and then once all your data is off break the connection from the guide.

Don’t forget to contact Apple to cancel your account so you aren’t billed for the service any more. Do consider your iPhone or iPad maybe using iCloud as well. So canceling it fully might be a problem. You may want to just roll back the size you are using.