I recently got a MIDI keyboard from synthesizers.com and I’m putting a raspberry pi 4 inside. I wanted a very solid keyboard with metal and wood enclosure (no plastic) and a high-quality keybed, small enough to fit in a carryon luggage and enough space inside to do the mod I’m doing. I want realistic electric pianos, organs (Hammond and Vox mostly), as well as some pads, leads, bass, etc. For now I’m running fluidsynth in the raspberry pi, that uses soundfonts (a sample-based format), and it sounds great. If you don’t want to do all the diy work, a simpler solution would be the original synthesizers.com keyboard plus a Zynthian box.
(Note I had to perform surgery on the rpi to remove the Ethernet port and 3 of the 4 USB ports, otherwise it wouldnt fit in a position to properly dissipate heat)