OS Update Testing - Cloned Internal Drive?

Hi!

I need to update my OS from Catalina to Monterey or Ventura on my MIni 2018.

PS and Logic X have new features I can't use without major os updates.

I've successfully cloned my 1Tb internal drive to a spinning platter external drive using Carbon Copy Cloner. (Wd 1 touch)

Their site recommends cloning then updating the OS on the internal drive for testing.

If it doesn't work, or otherwise then simply re-copy the cloned external drive back to the internal after you erase the internal....... yikes.

Im terrified. What if the re-clone or transfer goes bad?...or..I don't know.

1- Isn't it possible to just update the OS on the external drive, do some testing on it and if all goes well, then just update the internal drive with a new os?

(The new OS plays nice and my projects loaded ok etc)

2 - I don't really know how to tell it to boot from the external drive but I copied the Monterey installer to it so it's ready to go.

Do I run the installer while it's attached as external and then hope the OS finds it in reboot?

I tried to select the external drive as boot in the settings panel but it would not let me change it.

Is my theory workable?

Is the internal drive OK being dormant while I test out the new os on the external?

Is there a best way to do this?

Thanks for any guidance, this mini is my first mac.