There are plenty of similar threads around at the moment, but as the search function is buggered I’ll chime in.

For me the OT is master for clock and transport. I recently set it up to have pattern changes follow on all machines as it’s much easier to stay organised that way now that the a4 can save projects like the OT and the MnM has snapshots. Also in the midi sequencer you can assign program changes per bank or part however as you have mucrobrute and ms20 which have no patch memory in guessing it’s not an issue.

I have everything chained from the OT (tetra, MnM, a4 and v synth although usually I just sample the v synth as it’s too big to take out live).

I would chain midi from the OT to the ms20 and brute then have the A4 sequencing the 9U and routing the modular audio into the A4, OT input A B could be md and the brute and ms20 into c and D respectively leaving the volca to be sampled by the md?