Sorry I missed the part with the mixer. Sure, an external mixer would help.
I have an ES-3 and have used it for sound design and composition, but never for playing the case, mainly because I do not play a modular and use it solely for sound design and composition. Something like the ES-3 is good for communication between the DAW and the case. I also have a Mutant Brain MIDI-CV module that I had bought in 2019 for a specific project. I'll use that to send note/gate data to the case and use the ES-3 for sending envelopes, LFOs and also audio.
Personally I find most modular performances boring OR music that could easily be done using a laptop or other hardware boxes. Lots of reverb-drenched pointless ambient doodlings or never-ending arpeggios over 4/4 kicks. I think most people use a modular for the sake of using a modular - an extension of a pitfall in electronic music (academic or non-academic) - the tool or process is more important than the end result. I will, however, give myself a last chance at trying to play with a modular by focusing only on my Make Noise modules as a consistent, single-producer system.
If you want some more interesting, experimental uses of modulars in live performance and composition, I'd recommend checking names like Thomas Lehn, Richard Scott, Eliad Wagner, Thomas Ankersmit...
Electronic musician, improviser, composer, sound designer in Berlin
Solo, Böseblick, Ephemeral Fragments, Only With You, Pep Talk, Lettow Erel Duo