Have you read up about the rxmx and what it does? It's far from a normal mixer. It's sort of a hybrid scanner/vactrol gate. So, you send cv and gates into it to trigger certain channels over others in time. It's interesting in that's it's meant to hardware into the DPO or their fixed filter module, but it's not going to help with mixing and controlling cv stuff. Not saying that's bad, but maybe still plan to add a small basic utility mixer.
For vcas, you could go in one of two ways. First, a traditional VCA for both audio and CVs is really helpful and flexible. Remember that vcas not only control sound but any analog signal. So you use them to control the amount of a modulation source over time, gate it on and off, etc. For that, a VCA that can do linear (voltages mainly) and exponential (mainly audio). Fir that I'd recommend the Intellijel uVCA mkii. It has two channels, both of which can be swept between linear and expo, much like the two main channels of maths. Because it has a lot for that, and not just a switch like other vcas, means that you can fine tune the response to your liking. I would not recommend make noise's VCA offering (moddemix) as it's not meant to be 'normal'. It heavily distorts/colors the input sound which you may not always want.
Another alternative for a 'VCA' is to get a low pass gate module. These are vactrol based, and impart a nice, natural shape to sound. Input a quick trigger, and it decays the sound just like a plucked string, without even needing an envelope to control the decay. This is less useful for cv, but the ones that make noise offer (Optomix for instance) can still process it, just with that natural decay acting almost like a slew to the cv bring out in. Not always useful, but interesting. LPGs are the core of the west coast sound, especially combined with a complex osc source like the DPO. If you like those plucky percussive sounds that buchlas are known for, you might want to consider a LPG.