I did similar things with a customer many years ago when I was back in vendor space. The solution the customer came up with was to cache things out via a policy that cached, and then scope the macOS upgrade in Self Service only to devices that had the actual installer cached. I cannot recall the exact outcome, but I believe it was pretty successful. Their struggle was a global user base where some users were not in actual offices that had great Network/Internet connections so they decided to cache.
You should be able to build this with policies.
