Your original post is actually a very good starting system except for a couple tiny things things that seem off:
- What's going on with the 1010 Music MIDI splitter? Do you own or plan on owning multiple 1010 Music products or other TRS MIDI items? If so, then that's fine - just as long as you don't want it for anything converting MIDI to CV.
- You can probably start with one mult like that. This space will help you with my next suggestion.
- The Octal Linear VCA is great, and I often consider buying a second one. However, that one is better for big systems with lots of different modulators. I think here you need something with knobs for controlling the level and/or CV amount going in. Intellijel Quad VCA, Omnitone Ampera, Happy Nerding 3xVCA...something like that.
- I disagree about the Mimetic Digitalis not being useful in this context. Even when you're not using every part for pitch, it would be a good CV modulator. A Pam's New Workout might be a more versatile option to start with, though. Lots of clearly accessible modulation, gates, and triggers You can even take unused channels from it and send timed gates/triggers to effects or filters to ping them and create unique experimental percussion that will be good for complementing and thickening your exploratory sounds.
- For a small system like this I don't know if such a large output module is necessary. I have a passive stereo headphone out from Herzlich Labs and I find that way easier to just get the sound from one thing to another or listen to my system without needing a whole mixer or something - plus I only need one for all my stuff, rather than one per case. The 4MS Listen IO might also be a good idea, since it has the output functionality of this one (minus quarter inch jacks) but also has an input so you can run weird sounds through your filter and FM your synths with external sounds and stuff. If you have specific setup needs and really prefer the Pittsburgh output module (or already have it), however, ignore me on this.
In general, I think you had the right idea the first time. The second one was a bit weirder and might not really work since it no longer has an audio rate oscillator and that trigger sequencer is taking up a lot of space while doing less than the Mimetic or a Pam's. A 6U case (2 rows) is worth a look, though. The Mantis Case is cheap, durable, deep, wide, has good carrying case options, and will sell easy if you find it too big or need something else. Even if you choose not to expand much, it's better to have a case that's roomy and not quite full than be overly squeezed. There is generally never enough modulation or VCAs (though you want to make sure you have enough modulation to at least open as many VCAs as you have and intend to use, especially if you consider some without knobs later). Good work keeping a balanced and sensible amount of audio sources to modulation sources and utilities - that really is what makes this stuff come alive and be different from the results in a DAW or hardware workstation.