I really like your solution and your build - that’s some great work!
For what it is worth, I have also had some trouble with relay boards all coming on, just for a second, at startup. I posted about it here, and @glmnet posted a solution :Using PCA9685 to control relays: unit defaults to On
Basically his approach is to wire the relays in such a way that only one can ever be on at the same time. In the example he shows 3 relays, but there is no reason why you couldn’t keep going.
Just thought I’d share that info with you, because it seems to be a common problem with those relay boards.