There is a TSM command for reseting the initial user on tableau server tsm reset
Here’s the article on it tsm reset:
After the reset you’ll need to rerun tabcmd initialuser to create a new initial admin
Here’s the tabcmd initial user article:
And it looks like there’s a way to mass transfer ownership of workbooks and data sources.
Select all the ones you want to change the owner of and select actions --> change owner
Here’s the article on Manage Content Ownership :