The attenuator switches on the TAI-4 have nothing to do with the sort of attenuators I mentioned. In order to scale or alter CV and/or modulation signals, it's essential to have attenuator modules. Frequently, these also have the ability to repolarize signals...so if you want to send an inverted envelope to your VCF, and the EG has no inverted output, this would be what you needed. These also have mixer-type functions quite often, which then means you can mix different modulation signals to create bespoke composite modulation signals that can be quite complex.
Which gets us to mixers. You don't mix JUST audio signals in a synth. And, accordingly, there's two different types of mixers in modular (and most other) synths. One sort has a linear response, and this is what's needed for signals where scaling is important, such as with CVs and modulation signals. The other type (which you see often at the end of a signal path in a synth) are exponential mixers; since sound's apparent loudness (not signal level, but how WE perceive volume) follows an exponential scale, and these are pretty much solely for audio signal mixing. Plus, there's one other type of "mixer", and those are adders, which mathematically add signals together...so, if you wanted to transpose something up by an octave, you could have your CVs being fed via an adder, then use an offset DC source to add 1V to this...and there's the octave shift.
The 2hp VCA is fine for modulation level control. But since it's a linear VCA, it's not as useful for audio...unless you control it with an exponential-response EG. The real problem, though, is that far more VCAs than two are needed. You need two right off for controlling the output level and for any modulation uses, but since the rest of the synth is still very deficient, you can't see what the necessary VCA complement should be...but let me assure you, it's NOT two.
Stages is an envelope generator, true. But it has a lot more uses, and a lot of users use it as a CV sequencer of sorts. But in order to do that, you would need a proper envelope generator along with the Stages. Plus, envelopes are something that can be sent all over the place (along with LFO signals) to modulate other things; this gets us back into the "this build isn't complete" issue, since a proper build should have a lot more to it. But even with that, the Belgrad and the Verbos complex VCO are modulation-hungry things from square one, so having JUST the Stages isn't sufficient. Some proper AR and/or ADSR (better for filters and output VCAs) generators are sorely needed as well.
But the crux of the problem comes from the simple fact that this build is extremely incomplete. It's NOT a synthesizer yet. It's more akin to an incredibly expensive dub siren at this point. If that works, then fine, but I still believe you should build this out properly, especially after springing for some very expensive modules like you've done here. The difference would be like night and day.
Oh, and FYI...I'm a musician as well. But with 53 years of experience, 42 of which have seen me concentrating on electronics and electroacoustics, I might have a bit more perspective on this sort of thing. After all, they don't let you in the door to study with Stockhausen if you don't know what the hell you're doing...