Hi, Martin-Smith. I agree with what Portland has told you about setting a target in your registry for Windows 10 21H2 so that you just don't have to worry about 11 being foisted on you.
As to how I went about getting 12H2 on my old, slow desktop, what you suggest would not have worked any better, as it was not a shortage of disk space or slow internet holding me back. It was just slow internal processing on the computer (which I subsequently learned was due to a ton of registry errors that had accumulated over the 10 yrs or more that I have had that computer). I kept trying to download and install the update, and it kept stalling out. That made the software download folder full of aborted attempts that seemed to be acting like stoppers. They would have done that no matter whether I was downloading from the internet, a CD, memory stick or whatever. Once I cleared that folder out, the update showed up in the easiest, most "organic" way -- from the updates page in my settings. And now that I have used the scan and fix registry errors tool in Windows, it is running as well as it ever did. Life is so much nicer when one's computers are all working right!