I'm insanely jealous of your setup and retirement plan, but I use a flight case with a professional mixer mounted in it. I use a Roland M12-E with 12 inputs. I have my AWE64 on a separate channel and each MIDI box on a separate channel, with one output to the speakers.

For that many devices, I'd look to either a MIDI Solutions M8 Merger or a MOTU MIDI Express, coupled with an additional MIDI Solutions Quadra merger.

I'd also go full-rack, since you can play modules side-by-side and better use space that way.

Eidirol UM550 or 880

Nikanor wrote on 2022-09-17, 22:32:

How do you manage connections to the computer and keyboards?

Twin MOTU MTP AV+II networks (32 MIDI I/O ports)

So here’s how I do it:

If you can get an MPU-104 you can have multiple inputs, and then an MPU-105 for multiple outputs to the boxes. Then I run the output back to the mixer with one unified output to the speakers.

I use a Roland A-880 for MIDI signals, and a Behringer Eurorack Pro solely for MIDI sound outputs. Love the individual channel mute buttons on it, super handy.
Then I have a stereo splitter behind the Eurorack that feeds a channel on two Behringer mixers x1002b, to mix MIDI with other sound cards on different computers. Part of it: