One potential source of the long update installation time is going to be in how they ship firmware updates as part of their OS updates now and have done for a while (Note that they stopped shipping external firmware update packages for years).
Obviously there are always work arounds but we have to remember for 99% of users this is just the reality they're stuck with.
With Windows I'm doing regular updates and know that these take 15 min max. Make coffee and update is completed.
But with MacOS it is usually at least hour to complete update and it is always showing "2 minutes remaining" for 30 minutes...
There should be a hot key that can display the log of the software update process. That way the user can search for "MacOS update stuck on reticulating splines" to try to find possible solutions. I really don't understand why Apple makes it so hard to diagnose issues with their systems.
Fwiw n=1 my update to Big Sur went surprisingly smoothly in full clamshell mode connected to 2 external displays.
You could reasonably refer to running in clamshell mode as headless, but calling a system with external monitors connected headless does not make sense.
But I get what you mean.