The problem with making your output mixer too small is that it'll be a major PITA to use. You CAN find mixers in that 10 hp and down size that will give you stereo, but as you make the module smaller, you also eliminate the room on its boards for VCAs, autopanners, etc etc while, at the same time, giving you a very fiddly mixer with no CUE function (no room for it, remember?), no VCAs, and so on.
Again, this gets back to this "beauty case" problem. The mixer you want here DOES exist...in fact, there's a few of them...but there's no way you'll be able to jam one of those AND all of the synth's basic modules into a small skiff like the above. Or rather, you COULD...but the result will be very fiddly and annoying to use due to the tight spaces left for your fingers in amongst the knobs and cables. Personally, I would go with a larger cab and larger panels to minimize ergonomic issues than going this small, which will just maximize them.