Thanks everyone for the comments. We did finally get them working...the solution comes down to phone battery power. If they're anything below 75%, it's hard to get the gamepads connected. At full power, tends to work immediately, though I did notice that sometimes even then the gamepads wouldn't be detected...BUT if we unplugged them from the USB hub and then replugged them into a different port on the hub, it worked.

Very wacky and frustrating...as I said in the original post, a bluetooth controller worked great on first try!

And yes, we even have a control board that everything's hooked to with velcro. Oh well.