Upgrading macOS without Internet

I am now working for an organisation which has an 'air-gapped' network meaning Macs on that network have zero Internet connectivity. (Deliberately so for maximum security.)

Historically Macs in this situation could be updated by download a 'combo' updater from the Apple website and copying it to a file server in the air-gapped network and then having each Mac run it. Likewise a full-blown macOS installer could be used to upgrade them.

With newer macOS generations Apple stopped issuing combo updaters or even delta updaters. It is however possible to download a full-blown macOS installer via a script and it is also possible to use Apple Configurator to download the IPSW image file.

I would however appreciate it if anyone has real world experience of doing offline Mac upgrades and can indicate if there is still a need to be connected for activation purposes - which is the case when doing an erase and restore or any other issues that would make this difficult. Clearly connecting the Macs to the Internet to update or upgrade them defeats the point of their being air-gapped.

Note: It is not my choice that an air-gapped network is being used so advice not to do this will not be helpful.