To my knowledge there is currently, no direct PowerShell cmdlet or MS Graph API provided by Microsoft to list all the Forms created by a specific user or all users within a tenant.
However, Microsoft is aware of this limitation and there have been discussions and requests in the community for such a feature.
The only solution I can come up with is having users share their forms with a centralized account then manually address the forms in question. This can be an External address if you are not leveraging Cross-Tenant synchronization.