Digital synths are definitely DIYable too.
It just occurred to me that the open source Pushpin controller would be an excellent jumping off point.
Assuming you’ve got access to an old Push 2 ( and optionally ) some Elektron Overbridge gear, with a Raspberry Pi and audio interface, you then could concentrate on substituting for ( or modifying ) the Surge XT synth engines.
Any other open source sound engine might be a good candidate. I immediately think about the open source Terrain synth. ( post ) ( This is not the new Conductive Labs HW synth, but shares the name and is a similar algorithm. ) This is probably a larger adaptation but doable. Other options are in mind as well.
ADDED : Other options, as for example there are some open source physical modelling synths as well, that could be adapted.
This thing is sort of a Push 2 Standalone in a way, especially if you package it up nice.
This is also something that could be adapted as a replacement for the Omega ( thread ).