For now I focus on the Pi3B+ to get quick results with the least amount of efforts and without breaking the already sadly-looking bank. Pi4s draw and heat too much but could be used, as well as many other compatible devices. In fact the key is simply the form factor and in particular the 40-pins header. So "model A"s could be used as well, but I stick to the Bs because they have an Ethernet port that makes management very practical.
Now if I use As, the other management process will be through the Wifi link, which saves cables and a physical hub, but requires a dedicated Wifi manager: thing have only been moved around... But going further, Ethernet and Wifi will not be a great communication medium. The FPGA is already there to help the applications interconnect, so what about... exposing the link to the Linux system more broadly? write a block or character device driver ?
Going further, Linux will have little to do, such as managing the hardware health, configuring stuff, allocating memory and storage... No need of a full-blown distribution, right ? So I'm looking at "bare metal" systems such that the SBC can boot fast, avoid wearing off the µSD card, ... I can start with a basic OS image and remove tons of things, then add a custom application for exposing the interco. Unfortunately, because of systemd, long is gone the time when you could simply add "init=/bin/bash" to the kernel's command line :-(
I even wonder if/how I could let the whole system boot off the interco, thus saving the µSD card altogether. I know people have already made their own bootware, such as Tristan for the Pi3, but this normally goes on flash storage.
But for now and during development, I stick to the standard OS...