I have 8 Microsoft XBox 360 wired gamepads that I'd love to use to play Swordy.
I'm in the beta and submitted this as feedback but didn't hear anything in response so I searched around and found this forum.
You mention triggers behaving weirdly when using DirectInput for XBox360 controllers. This is not a Unity issue it is Microsoft's intentionally flawed drivers causing the issue. When accessed via DirectInput the gamepad's triggers are on the same axis and rumble is not available.
The way most games do the switch is having a togglable option to switch to DirectInput even for XInput controllers. (Some games require you to restart the game after making the change.) A handfull of games switch seemlessly in the background but that would confuse users since triggers won't work the same.
I wouldn't mind having Left Action / Right Action moved to the bumpers and pickup moved to one of the regular buttons A,B,X or Y if the tradeoff was more than 4 XBox360 gamepads.
If you're using InControl to access the gamepads there is an option to switch to DirectInput for XInput controllers (according to documentation), and I've heard ReWire does too.
I see no trace of SDL2 among the binaries but that also has options to use DirectInput.
Please reconsider supporting more than 4 XInput controllers. For me it is a buy or pass feature.
If there is anything I can do to help let me know.