macOS Big Sur updates - location of files?
Welcome to the site.
First of all, I can very much empathize with this situation. For years we lived behind a slow satellite connection which had a quota of 20GB per month.
Unfortunately, the bad news is that softwareupdate is no longer supported by Apple for system updates in Big Sur. I am not even sure that it is possible to do what you are trying to do. Apple used to post ‘combo updaters’ to their website, but they have also ceased to do that.
One of Apple’s perpetual blindspots is that not everyone has access to fast and unlimited Internet access.
Your best bet is to set up “Content Caching” on your local network:
What is content caching on Mac? - Apple Support
Set up content cache clients, peers, or parents on Mac - Apple Support
(Note: I have had some trouble loading some pages on Apple’s support page in Safari. It works sometimes and other times says that the page does not exist. If that happens, try another browser. Yeah, I know, absurd.)
Content Caching is not as easy or as precise as combo updaters were (for one thing, I don’t know of any way to force a Mac to only use a content cache… it’s just supposed to happen “automagically” and we know how that goes.
I wish I had a better answer for you. Unfortunately, Apple has chosen to deprecate softwareupdate and discontinue Combo Updaters, which makes this situation extremely difficult to manage, especially with bandwidth restrictions.