Hello and welcome!
I agree, the amount of choices in Eurorack is very overwhelming, and I have found that the hobby has great potential to take me away from the process of making music by encouraging me to constantly reconsider my setup and its future.
I have taken a similar approach to you and focused primarily on Intellijel modules. After about 1.5 years of patching, I now realize that Intellijel’s easy-to-understand designs really appeal to me. Where I feel other Euro companies tend to sometimes take a rather complex, esoteric approach to design, I appreciate that Intellijel has always seemed, to me, to lean more toward the fundamentals. That leaves a lot of the creative process in my hands, from the way I choose to use the modules to the way I patch them.
Looks like you have a nice two-voice starter setup you’re working on. One thing missing is envelopes. The Shapeshifter has a decay feature, so you could get a (limited) envelope that way, but I think it’s only for oscillator 1, not both. For envelopes, Intellijel has ADSR and Quadrax, though I think they will be hard to find at the moment.
As for USB MIDI, not sure what you’re proposing to do but from what I’ve read, the expander doesn’t connect to the ins and outs of the case, it connects directly to the USB output on the back of Metropolix.
EDIT: @svarta.kusten corrected me below, apparently the Mixup does in fact connect directly to the 1/4" outputs of the 7U case. However, all the Mixup manual says about the signal is that it will be “unbalanced” – is that Euro or line level? I would guess Euro like @svarta.kusten, but to me, “unbalanced” suggests line level…though I have no idea how the circuit board for the jacks would achieve this…?