Super interesting.  I've been thinking about doing this recently (making a mechanical keyboard with domes for a small handheld), but I keep convincing myself that anything I'm likely to be able to create for the membrane would be unlikely to be of high quality, or give a good keyboard experience.  Have you thought about using a "found" membrane, like from the Rii mini keyboards om Amazon, and just building your device around it?  Obviously non-ideal in that it has a weird dependency on parts from another device, and it's not scalable... though I haven't really found off-the-shelf mini-keyboard membranes anywhere.

I did ask makerfabs (the company in china that builds my boards) if they could source any membranes on their own, but they didnt seem to be able to find anything useful. The snap domes are available in a range of strengths from 55g to 250g activation force, so there is the possibility of tweaking things there. Then there is the membrane design for the top. you could tweak the thickness of the rubber to change how spongey it is. I found out the other day that PCB Way do a short run vacuum casting service for things like key membranes which might be fun to try too

These look great! What technology are these printers using? SLS?

yeak its SLS. The guys at Midlands 3D got a machine specificly for it, otherwise they would have to keep stripping down the machines for a deep clean to switch between the TPU and  the Nylon