A swap for the Unify could be the Erica Synth Black Stereo Mixer.
Or a Synthrotek Stereo Output Mixer (which has no CV control over panning)
I recommend reading a bit into a few Muffwiggler forum posts. This one is quite interesting:
https://www.muffwiggler.com/forum/viewtopic.php?t=154186
I am unsure why a multi-channel stereo mixer makes sense for you? Do you plan to mix in external sources?
If it is just for the sake of a output module I would get something smaller...
Apart from stereo (synth voice) mixing I would recommend, getting dedicated VCA's (like the µVCA) and dedicated small mixers (like Mutable Shades or Intellijel Triatt).
The optomix is a combination of the above (+ a few features), a 2-channel mixer with VC Amplification per channel and sound shaping (filtering) features per channel.
The Intellijel Linix or the Planar, also are similar in the sense that they're a combination of VCA's and mixers.
For a 2U system with two voices (like yours), with using the above mentioned modules I would get (at least):
1x µVCA (which are 2 VCA's)
1x Triatt (which is a 3 channel mixer)
1x Optomix (which is a 2 channel mixer with VCA per channel)
You can get more VCA's later