Thanks. Yes, sorry, I’ve had lots of practice.
I’ve seen a discussion about Jigdo for Raspberry Pi OS, but I don’t think it is a good match, and they concluded it wasn’t available. In my experience, Jigdo works best with ISO9660 installer images that contain a lot of large package files, rather than the tiny files in their installed state. Jigdo is also really difficult to explain, so the investment would not be returned.
However, I don’t see why it couldn’t be attempted, using (again) a remote virtual machine on the internet side of your expensive internet link.
You could try talking to the Raspberry Pi Foundation about it. But most people resourceful enough to get a Raspberry Pi are also able to get images, even if they are preloaded SD cards by post, or through friends with bandwidth.
(I wondered if rdiff could also be useful. I just tried it for the transition from the Raspberry Pi OS Lite image from 20th August to 2nd December; the signature was 10 MB, compressed to 6.6 MB, the delta was 343 MB, compressed to 128 MB, the original file 438 MB, so the saving was 303 MB, or 69%. I ended up with exactly the same .img file on my side of the network link, at a “cost” of 138 MB.)