Well, if the objective is to keep this very West Coast, I'd yank the DPO altogether and put in a Maths instead. Since Maths is a complex-wired version of what's basically a Serge Dual Universal Slope Gen, it fits. You can use the two slope gens as modulation, or you can cycle them at audio rates for extra VCOs. Lose the buffered mults; you don't have enough tuning-critical signals in here to justify their expense, and the rig itself is small enough that inline mults or stackcables make a lot more sense here. With those removed, you then might want to put in a 2hp delay to go along with the 2hp reverb (since reverb was key to the Buchla sound, and delay [via the Wilson Analog Delay] was a significant part of the original Serge sound). And since these simpler modules are "appropriate", then you can remove the DSP as being more or less redundant.
Another redundancy might be found with the A-149-1. The Wogglebug is actually an augmented version of the Buchla Source of Uncertainty, so having two of these is a bit much. The A-149-1 is also WAAAY too deep for a Rackbrute cab; you want to keep your module depths around 40mm or less so that you can have space for your ribbons and airflow inside the cab. With that gone, you might then want to add a stereo filter to tone-shape your mixer output; while VCFs that are typical to subtractive synthesis aren't 100% "West Coast", adding something that can act as an overall timbral shaper would be right. Have a look at WMD's Overseer, for example. It's not a "normal" VCF, but if you used it post-mix as a timbral control (either manually or modulated) that would be more in line with what the Buchla 291 is for, especially since you can CV the response behavior which makes it act more like the 291e's "morphing" capability.
Last, something additive as a VCO. The Verbos Harmonic is nice. It's also HUGE and as such, isn't really appropriate for a small build. The Mannequins Mangrove, however, is only 10 hp and while you can't directly address individual harmonics, you still have quite a bit of leeway in manipulating harmonic content with it.
This should be a lot more spot-on with the West Coast model. I am wondering about that MScale, though...if this is in an Arturia cab, wouldn't it be linked with a Minibrute 2 or 2s and not a Moog? Or does the MScale deal with a Moog that's somewhere else in your rig?
The two complex oscillators cover a lot of the same ground --- I'd swap one of them out for something different like a Verbos Harmonic Oscillator or a Wavetable Oscillator.
Thanks richc90 for your relevant comment. Due to the relatively small size of the RackBrute 6U, I would opt for a Wavetable Oscillator as the E-352 Cloud Terrarium or the Erica Synths Black Wavetable VCO. And alternatively Lugia provides a perspective which could be great too.
Thank you so much, Lugia, for your comprehensive and detailed suggestions that I have integrated in the new rack below. You helped me in a previous thread to set up my very first modular around semi-modulars and sequencers I already own. https://www.modulargrid.net/e/forum/posts/index/3864
This current thread is just (at this time) a mental exercise, though potentially achievable later within a second rack (end of the year?) The reason is I recently rediscovered Suzanne Ciani works, including the Buchla Concerts 1975, and I'm mesmerized by this music!
MScale is paradoxically included here due to the Mother32 already on my desk.
I finally swapped the A-138s for the new Make Noise X-Pan which apparently seems more in the quadraphonic philosophy of the Buchla universe.
PS: the RackBrute depth could be a bit larger. "53mm on the upper part (power bus) and 75mm on the lower part (no power bus)". https://www.arturia.com/faq/rackbrute/rackbrute-general-questions
But I'll verify that point very soon (delivery is scheduled for next week...)
Dove has a couple of great digital modules out or on the way. There's the WTF Oscillator which uses some pretty neat calculations to combine two wavetable waveforms. There's another that I can't remember the name to which has four wavetable engines and uses two pots to move a small crosshair representing your mix of oscillators (basically emulating a joystick with etch-a-sketch controls).
I'd really consider one or both of them for some West Coast luvin'.