I have been thinking of splitting my rack into 2 as well, using one as a controller for the other. I was going to use the new Intellijel Octalink for linking the two systems up. Since you don't have any 1U, in 3U scale there is the equivalent Doepfer A-180-9.
For routing/mixing there are many many options.... Here are two options that just happen to be too big for my rack but seem to be really popular:
https://www.modulargrid.net/e/wmd-sequential-switch-matrix-ssm
https://www.modulargrid.net/e/4ms-company-vca-matrix
Personally, I am going for something smaller:
https://www.modulargrid.net/e/joranalogue-audio-design-select-2
In my rack I use 2x Quadratts for mixing, but since you don't have 1U, you might consider getting a couple Shades or Triatt modules. Or do what @Lugia was saying and look at some of the rad 8x stuff that Erogenous Tones makes (like Levit8 or VC8).