I tell all my mac using friends to always keep a second admin account. This would have saved you easily.
Also, I have to say this, not to chastise you, but as a warning to others: DON'T ever try to change your username.
The moment I read "no admin account" I knew that this is what you had done. We have seen many many cases of this.
Setting that aside, the traditional way to solve this involves fooling your mac into booting as if for the first time.
This technique does NOT work in Sonoma (*), but we have an arguably better solution.
1) BACKUP - this is crucial!
2) You could try Erase Content and Settings - this would have your mac start fresh, then you could create a new admin account, and subsequently migrate your account from your backup
BUT... you see the problem? I don't believe you can do Erase Content and Settings from a standard account...
So I am afraid that the solution now may require
1) BACKUP
2) FULL reinstall from recovery (and creating a new admin)
3) Migrate your account from backup
4) Use the other admin to regrant admin privilege to your account
(*) As seen in this documentation page: What's new for enterprise in macOS Sonoma - Apple Support
- Removing /private/var/db/.AppleSetupDone no longer relaunches Setup Assistant if a local user already exists on the Mac. Erase All Contents and Settings can reset the device and launch Setup Assistant.