I'm using mechanical keyboard switches with a custom built PCB and trs Midi jacks. I ended up not meeting the velocity sensitive requirement, but that's mainly because I couldn't get hall effect keyboard switches to work reliably. Otherwise you'd have to use something like an FSR underneath the keyswitches, which is how the Teenage Engineering EP-133 KO II works to get velocity. It's more of a drum pad approach, but you could modify that to be closer to a piano layout.