SELECTED ANSWER
replied on January 29, 2024
I don't know if you can prevent the export from including the Starting Rule, but you're not required to bring the Starting Rule back in during the Workflow import.
When you're in the Starting Events part of the Workflow import, under "Action" for the rule select "Discard". The Starting Rule won't be imported with the Workflow and thus can't overwrite/modify any existing Rules.
We've just filed Cloud Workflow user story 507094 "I want to be warned when importing a starting event would disable an in-use starting rule" so hopefully in the future you'll get a heads up. Because Cloud starting rules and their support for many:one attachments work differently than self-hosted Workflow's 1:1 starting rules, it definitely makes sense to throw a warning in the import process if you're about to make a change that impacts existing workflows.