So, for a single purpose kiosk device, I've had some success with not using a DE or WM at all - essentially putting the desired application inside your xinitrc. In that case, you would need to configure the window geometry using xdotool (I think). There's probably something similar for Wayland. Alternatively, a tiling wm like i3 or Sway could be useful if you remove all the keybinds from the config file.

ETA: for a device like that, I would say that arch is a potential starting point, but you might also consider an immutable distro. I don't have any experience with them though.

ETA2: For the hardware, a Raspberry Pi is not a bad starting point, but Arch on Arm isn't officially supported. Your mileage may vary

It's hard to give concrete advice without knowing the specs or the software you want to run on this, but for tiny Linux systems there's Buildroot so you can compile just the bare minimum you need and not use a distro at all (unless you could Buildroot as a distro). This is what OpenWRT uses to build all the router firmwares among other things.

For something that would go in a car that seems pretty ideal to me. Skip initializing things you won't use, make something that boots to GUI in 3 seconds. When you want to update the software you flash it as a new firmware image, no on-device installing or anything.

Depending on what you run, ideally you'd skip Xorg/Wayland and use the framebuffer directly. But if you need to run a more standard environment, that's what things like Cage are designed for. Single app, always full screen. It's called a kiosk environment.

GitHub - cage-kiosk/cage: A Wayland kiosk

A Wayland kiosk. Contribute to cage-kiosk/cage development by creating an account on GitHub.

^GitHub

i've done this using android and linux.

i've repurposed my old androids to turn them into wireless cameras for my home made security system based on the motion project as well as created a homemade lojack and dashcam for my car. some phones i was able to root and i picked and chose what software ran on it and the others, i disabled as many services/apps as i could to make them as minimalistic as possible.

for linux, i created my own home made router and wifi access point; but it's kind of the opposite of minimalistic as it's a full blown linux server with virtual machines and desktop environment to help with management.

i provided both extremes to mirror what others have said: it's hard to provide advice without knowing what your goal is.

Small tangent, I didn't have the energy to read your whole post, so you might have addressed that. But often it's cheaper to go with an established multi purpose device instead of building something new.

I used to build and sell Raspberry Pi gaming handhelds that were as cheap as possible and literally just held together by some string. My purpose was to get enough money through the sales to be able to build one for myself. Sure, the building process was fun. But when I crunched the numbers just buying a cheap smartphone and controller was much cheaper and more performant and versatile than the self-built solution.

Just buying a cheap phone or tablet is often the cheaper solution.

Heck, even Valve just bought off the shelve tablet displays and turned the image in software for the Steam Deck.