I should've also said what I did about buttons -- probably useful

I have a similar setup, only without the explosion of misc buttons. You might want to think about what you really need, especially given that the U360 supports shifted buttons, as do the iPacs.
My trackball and spinner are both USB connected, so they go straight to the PC.
The P3/P4 U360s are managing the 4 game buttons, the player start button, and the "admin" button (which is really just "pause" for HyperSpin -- HyperPause has a lot of the admin controls in it already). The pause button is a shift button for additional functionality on the 4 game buttons and admin button.
P1/P2 U360s are not running buttons. I have an IPac Ultimate which the P1/P2 buttons (8 each), plus the P1 start/P2 start and shared "pause" button go into. Additionally the 4 "coin in" buttons and two pinball flipper buttons are on the IPac Ultimate. The IPac Ultimate is controlling most of the RGB LEDs, but I have a PACLed64 handling a few straggler LEDs.
I have a Bluetooth keyboard for doing anything more complicated than that.
There really aren't any 4P MAME games I found that need more than 4 buttons. Some consoles had more, but most console games kinda suck with arcade controls. For those I have a couple USB ports for wired Xbox One controllers, which can be used instead of the controls on the cab for games like that.
And, like I said, Wii games use normal Wiimotes over Bluetooth.