I've been hard at work in the last month or so, redesigning all the boards to overcome the issues I found in the first revision. Last night I got a message they were finally drop shipped, and hurried to get them. Just in time! Hopefully by the end of today I'll have some assembled, working boards.
I've also added two new boards - a modular potentiometer expansion board, and a modular pushbutton expansion board (on the left). I got the parts for these from some sellers I found on AliExpress, since I needed there to be enough parts for making an actual interface and the prices were a lot cheaper than getting pots and buttons from Digikey. Sadly, that means I'll be waiting quite a long time for the cheap shipping option. I sure do hope the parts will get here in time. If not, I'll have to come up with some other plan for a working prototype.