How to create an admin account without having an admin account locally

You can use the instructions in the following article to create a new admin user account:

Restarting Setup Assistant to create Admin User Account - Apple Community

Then you will need to follow the instructions within the link @PRP_53 provided to fix your main admin user account.

Unfortunately, you may have not followed all the instructions when changing the User Account and the Home folder associated with these actions

Change the name of your macOS user account and home folder

AFAIK - what you asking to do may not be possible

You may need to Start Over from Scratch

I am away, in some circumstance, you might need to do some technical things that may allow you to run Setup as if the machine were brand New

But I am not 100 % of how or if this would work

👍