You hit the nail on the head with your questions. Without quoting, I will just
provide info.

I looked at Buildroot and yocto, and those are, as you said, for single-use
purposes. I want something more flexible and generic, so that multiple things
(functionality) can be built using one basic configuration. More below.

Security is not an issue at the prototype level, but if I move on with what I
want to make, it can be a component of it. I am only seeing updates as being
possible by putting the device into a particular mode, preferably when it is
"tethered" to a general purpose computer to push updates in it. This is like
flashing a firmware with updates, so I am not worried that it cannot be done
and generalized in the future and while in production.

The pinball example is simple, but think of my Intel Euclid functionality. I
want to be able to drive different hardware components from the device, and
this will likely be with a processor that can handle the computations. (Euclid
gets pretty hot and goes through some serious power for what it does, for
example.) Some of what I have in mind requires this and some not. Simple sensors
will likely run on ARM, but more serious processing will have to be done on x86.
In that regard, I need a unified distro and toolchain around my process, but all
based on a generic "embedded distro" that is customized and customizable by me.
This just about sets 50% of the requirements, and the most basic ones at that.

You nailed it with the GPU question. Think of real-time 3D graphics processing
on board, while networked, etc. Choices of GPU hardware are limited, but they
are there to purchase for mobile systems (laptop-like). "Embedding" drivers on
a custom distro may be my biggest problem.

Andrew, you are my man for this brainstorming; you are asking all the right
questions. Happy to see this level of expertise locally.