My 2 cents-
You already have the orange pi's, I would think step 1 would be to just mount them together via standoffs (or not at all), hook up power, connect to a switch via ethernet, get them booting, working together, etc.- then see what you have. If all works as planned, go to step 2. Step 1 is 10 times easier than step 2, and you most likely have everything needed on hand to do it. (You could make a video about it- would give viewers an idea of what the end result is all about, and if step 2 never happens for whatever reason at least you will have had something to show).
You will find out what is important, what is not, you will get actual power measurements, heat measurements, what works, what doesn't. You may discover many things that could be useful to know. Let the set of pi's run for a week and see if they are what you expect- maybe they are flaky and full of little problems, or maybe they are great- you will at least get a good idea before putting in a lot of time on a board design.
Designing a board before you know you have a working 'circuit' seems like a 'trap for young players'
Just a thought. (There is still 2 cents left, so must have been free advice).