What is your take on this
Here's my main rack (actually two, both 6U, the bottom is an L-shaped Doepfer base):
On the bottom left corner is an Erica Synths Black Stereo mixer that accepts four stereo pair 1/8 modular inputs, and
outputs to the (originally mentioned in my first post here) Intellijel Audio I/O as a final stereo mix. The Audio I/O sends the attenuated stereo signal to my external mixer. I'm like you, in that I don't have much use the for the left side of the I/O, tbh, although it's there if I want to bring in semi-modular 1/4 outputs or loop a guitar pedal --
for my other rack that does just that, I use this:
https://www.modulargrid.net/e/boredbrain-music-unifx
There's probably a smaller hp solution for what I use the Audio I/O, but hey, you work with what you have!
I have fiddled around a long time to get the main rack I want, taking cable distance, work flow, and all that into account,
and am happy with this. Ideally, the ES Black mixer would be next to the Audio I/O, but I tried that and
for some reason I've forgotten it didn't work for me . . . note that I have three dedicated sub-mixer modules in
the center of the rack to feed into the ES Black mixer, with L/R signals of each taking up one of the four stereo pairs, and leaving a stereo pair vacant for whatever needs special attention. But that's overkill for your system!
You can always expand 8').