By using a MIDI to CV converter you can connect to your rack as if it is a typical hardware synth via MIDI. This allows you to connect any MIDI device to it (controllers, DAWs, stand-alone sequencers, etc.). If you go the USB to CV route, you can use your DAW as a piece of modular equipment. But then you're limited to addressing it AS modular.
MIDI-CV: less expensive, easier to use, universal
USB-CV: more expensive, more difficult to use, very specific
You will also find, especially if you're using a PC that the USB-CV solution wants to be considered an audio interface. PCs do not like multiple interfaces directly connected. Macs are a little better as they can aggregate multiple interfaces.
I'd go with a MIDI to CV solution for such a small rack as you'll have fewer headaches. You can always add an ES-8 (or even better an ES-9) to a larger set-up where you can get more use out if it where the trade-offs are better.
Getting rid of the VCA? Hell no! :)
You need VCAs. You can get modules with more VCAs in HP8. But what you have here should be adequate with lots of extra features like mutes. I'd keep it.