ATC -
I'm wondering if this would work.
Create a "Fake file" to trick software update into thinking you ALREADY HAVE a copy of the Monterey installer in your Applications folder.
"How?" ... you're thinking.
Here's what I'd try:
Open the Text Edit app.
Create a new text file.
Just put a few characters in it (for example, type "fake")
Then, save it to the desktop for the moment.
Now you have some investigating to do.
What is the file extension of the Monterey installer?
I'm thinking it would be ".app" (for application).
So, you want to do the following:
Rename the text file to "Install OS Monterey.app" (it would have to be the same name as the Monterey installer, with the .app extension, too).
NOTE: I don't have the Monterey Installer available to me at the moment, so I'm GUESSING that the proper name for it is "Install Mac OS Monterey". If it's not, make the changes as needed.
Once you rename it, do this:
Click on it ONE TIME to select it
Bring up "get info" on it (command-i)
Put a check into the "locked" checkbox.
Close get info.
Now... the finder can't modify the file.
Finally...
If there's a copy of the Monterey installer sitting there, move it to the trash and then empty the trash.
Then... move your fake file into the Applications folder, taking the place of "the real" installer.
Now I'm thinking that when software update "looks" to see if you need the Monterey installer downloaded (for whatever reason), it will check the Applications folder, find a file there named "Install Mac OS Monterey" (or close to that) ALREADY THERE, and then... just... stop.