Very cool work-in-progress! I spent a good chunk of time on a similar-ish rack concept, though mine focuses more on sampling, because I wanted to replicate some of my favorite aspects of the Digitakt while adding some more front-panel control and a few bits of Octatrack functionality. I decided to keep it within 6U 104hp to fit into a Mantis, and it ended up being a really interesting challenge. If you're interested in seeing where that took me and what modules I ended up choosing, it's all here (still a bit of a work in progress, as all racks are): 
Regarding the mixer part, I ended up going with a couple modules from Tesseract Modular's Tex Mix series, and I absolutely love them. As far as I can tell, it's a one-man shop in Spain, and the guy is super friendly and helpful, so I try to mention his stuff anytime mixers come up as a topic. You chain the modules to create the custom mixer of your dreams, and they're all very affordable. I went with the Slider Man module to get some fader controls into the mix, and it works like a charm. :)
PS: I didn't realize it would auto-embed. If that's messing up the conversational flow, let me know and I'll mangle the link to keep it from filling so much vertical space.