So, I’ve got a vm setup to booth and do steam auto installs with steam running periodically. It can be set to offline mode and in such a mode, if another machine on my network needs to pull the install it will do so locally from that vm without going on it to the internet. If I block external access, again steam will pull from that machine to install on my main gaming machine. Periodic backups of the machine makes sure that I have full installs ready to go for any of my truly offline machines.

It’s actually pretty cool to get gig speeds installing something from steam because it’s already somewhere on my local network.

It’s not as nice as GoG though. Definitely recommend that method if you can.