Mixing with an external mixer will only work well if you have a mixer that can handle the signal levels that are used within a modular synth...typically for Eurorack, that would be zero to +/- 8-10 volts. In contrast, mixers usually want to see either .775 volts (the "consumer" standard) or 1.2 volts (the "pro" standard). See here: https://www.kfs.oeaw.ac.at/manual/3.8/html/userguide/461.htm Neither of the "normal" level standards are anywhere near the much higher levels in modular synthesizer audio paths.
Since the idea here is to send the signal to the computer via USB anyway, you might find that something along the lines of the Expert Sleepers ES-8 would be better suited to this. Not only does this offer four CV/gate/audio inputs and A/D conversion, you also get eight outputs which can be CVs, gate/triggers, or audio, depending on your needs at the time. With this, and perhaps a stereo performance Eurorack mixer that offers CV control over levels, panning, AUX send/returns, etc, you would not only eliminate the need for an external mixer, you would be able to directly link your computer bidirectionally via USB for control AND audio. And also, eliminate the worry over level/impedance mismatching between an external mixer and the modular.