Hardware Polysynth(s) .... Preferred Physical Interconnection DIN - MIDI vs USB ??

I know you said to disregard, but the preference by far is to use MIDI DIN.

I use a Kenton MIDI merge to merge the clock from my Midronome and the MIDI note output from my audio interface, and that goes into a Kenton MIDI Thru (there's devices like a Mio XM that do it all, but Kenton is simple and rock solid).

I have a return as well using another Kenton MIDI merge for a few things that I want to send MIDI back into the DAW.

My Keystep sends MIDI into the computer via USB, though. But I prefer to have all the outbound MIDI+clock flowing through the same devices for the tightest timing possible.