System Extension error at startup

... I still don't see an option to enable system extensions

There is no such option — and it doesn't help that Apple muddies the waters by retroactively defining what had been known as "kernel extensions" as "legacy system extensions" — but no matter.

You may want to review some of this Developer discussion which addresses the bug I alluded to earlier: Mac OS Big Sur Preference error when trying to launch security preferences to Allow System Extension or Kernel extension | Apple Developer Forums but please be advised following those actions will be "unsupported" and could be irreversible.

In other words you're on your own trying to fix it, but hopefully that's something you already know. Apple does not make it easy to "undo" a macOS upgrade and that's not getting any easier. It's becoming like iOS.

the only thing that was to update was Safari...

That doesn't make a lot of sense to me. If it was your intent to "undo" the Ventura upgrade, it seems to me the only way to do it would be to completely erase the startup disk and reinstall that Mac's originally installed operating system. Then you would need to repopulate your User Account data from a Time Machine backup. Updating only Safari doesn't seem right, and could be a contributing factor.

This was a bug in an earlier version of macOS that had since been fixed, but I'm confused about something.

after reinstalling the system (Monterey from Ventura ...

I am not sure I understand. Did you completely erase the system running Ventura and reinstall its original one — Monterey, presumably? If so, did you subsequently update Monterey to its latest version?

... further more, the window doesn't allow me to click on anything other than the close button,

Including the lock icon?

The way this is sounding makes me suspect that original bug was reintroduced by effectively downgrading the system. I don't know if there is a way around that.

Thanks for that clarification. I believe the Safari update was merely an update for that app, which is ok.

We can no longer restore an entire system. It's the iOS way; hence, the future of macOS — if we're not there already. I have been somewhat surprised by the fact Time Machine continues to exist.

It is... I mean... who comes up with these ideas?

Preaching to the choir brother Al. It's... "for your security..." 🙄

Been there, done that too 😅