Do you have these on hand already, or is this still speculative? The reason I ask is because the key to getting something more playable is going to be starting with a bigger case, something like a Tiptop Mantis. And this isn't simply because of the usual routine of "go bigger, then shrink your build", but because it sounds like you need more interactivity in the controls, and trying to do that with tight spacings and tiny controls won't be any fun.
If this is designed to complement the yangqin as something of a "sidecar", you should make the modular as playable as the yangqin is in the rig. The idea here is that the modular is supposed to function as part of that instrument, so you really won't like having to go from a really obvious musical environment (the yangqin) to a confined and twiddly one. Rather, everything should simply "flow", as if this were all one instrument...because technically, IT IS.
Now, as for the pickups...this is another place you need to be careful. Piezos tend to have low outputs, so it might be a better idea to boost the signal level BEFORE it arrives at the synth. Mission Engineering has a volume pedal, the VM-PRO, which has an option for piezo inputs with the proper impedance level (very important! piezos typically like to connect to something high impedance) and boost for those pickups. Plus, this will give you a few new playing methods by default, such as manually reshaping the envelopes while playing, or striking a large number of strings and then gradually bringing up the level of the resulting tonal wash, all by having the volume pedal between the yangqin and the synth's input. Plus, with the VM-PRO, that Doepfer A-199 will be a lot happier with the incoming levels, as this won't require the major level boost that a typical piezo pickup does. This is sort of familiar turf, as I've started (again) using guitar (dual-neck lapsteel...in this case, a fine 1952 National Dual-8) in a few things, and this seems to work better into a synth if the levels are already up near line-level at the synth's input module.