I think external modules would be fine! As users, we can build little 6" cables if we need to as long as the connectors are easily available - or you could even include them with the kits optionally.
It's always discouraging to have to toss work that you've invested time into - but as engineers (I do software, not hardware) - it's important to know when to cut losses and just start fresh with a different avenue.
Do you have a github repo that we could sponsor, or anything like that?
external is fine but just be cognizant that it adds a lot of new complexity and failure points. Also cost of each enclosure can be a problem unless you can spec into something super available. I’ve had a number of diy projects where the enclosure and faceplate cost more than the electronics and that’s something you really want to avoid.
IMO the problem with most available Controller hardware is that it’s not designed for failure. Any single failure can result in almost catastrophic runaway and any time you add additional failure modes it’s becomes even more complicated to design around. On one board you can have secondary monitoring once you get away from that architecture everything acts independently and you need a whole new set of failsafe logic.
you are not worried about house panel circuit's tripping ? these test setups will draw almost near capacity (20A is the normal limit ?) , i think the dryer outlets are 40a/30a