Update:
I got that second Mantis, ignored Jim and got the brackets, didn't like how big and unfocused the whole thing was, realized Jim was right about the brackets, panicked at the flippancy of my approach, returned the second mantis.
Another few months of trial and error and this is what it's looking like:
Happy to say, it's finally starting to feel coherent!
Nerdseq means I don't have to drag the KSP around anymore if I don't want to -- plus I already have experience with trackers and the extra envelopes and lfos free up the rest of the rack considerably. Vortices streamlines the decentralized mixing I had been doing, plus acts as extra VCAs; Tides gives me access to complex / interrelated envelopes and LFOs when I want them; Select 2 neatly fills a few utility holes in minimal hp (offsetting, switches, "hang" circuits, muting); and those tiny VCAs (most recent pickup) allow me to fit in the Befaco Out, which it turns out I really don't like going without.
I'll be picking up a 3Ux84hp skiff this week as an overflow case and I think that's where I'm gonna park for the long term. I'm loving the main rack, and the overflow will be enough to let me keep OPTX and a few other modules of interest sitting around without being tempted to let it grow out of control. It also seems fun to make portable mini systems every now and then (out of stuff I already have).
Thanks again for the help getting over that hump!