I'm thinking that I might be better off to try to "cluster" a Pi Zero into the mix, use it to handle the display, a camera and maybe a couple minor sensors. Mainly because that'll probably make the display section easier to set up.
I've searched for and found web pages describing how someone or other set up multiple Pis in a cluster; that's not what I'm looking for. So, does anyone know of how I would set this up? Wiring diagrams for what goes where and how, software commands, that sort of thing? My battery power supply can provide power to two separate items, so it's perfect for this.
9/12 - bought a dual-fan cooling system for the Pi 4b. Takes it from at least 62C to 35C (aka 143F to 95F) Makes a big difference, and it includes a full and labeled 40-pin connector, so it doesn't gobble up GPIO space.
I've given up on the PI-Zero cluster idea, now that I have a better idea of what that actually is. Still need to find a display. The one I liked has become less likeable. Adafruit has one that looks about right, particularly since it uses the little A/V connector on the side, rather than the 40-pin. The 2.5" size may or may not be something I can work with; the 6v power requirements are something else. Hoping to get a line on a simple way to step it up.
I've acquired a PiMoroni Phat Stack, to make use of multiple devices. got a Breakout Garden, to mix & match things. Found a chip and a board which may or may not allow me to extend the 40-pin, sort of.