My current company keeps trying to cook up elaborate systems to keep certain deployments from happening while others are going on and I couldn’t recall ever having to solve this previously which is odd because of course this has been a problem before.

Yeah I was using my CI system to handle the CD constraints and it was so straightforward it hardly registered as work. I was setting up one build agent with a custom property and all the builds that couldn’t run simultaneously would all require an agent with that property. So they just queued in chronological order of arrival. Done. Next problem.