I don't know about the size etc - that's something that you'd have to work out for yourself - do you like trimmers instead of pots and poor ergonomics?
DIY is definitely possible - but I wouldn't recommend it as a first smd or even first mutable DIY
I would take a look at amazing synth and work out a path to building one and remember you'll also need a discovery board or similar to program it
However, I see that there are some pcba versions (smd pre-soldered) available - I would go for one of these - should be not much more difficult than the prok drum modules you have built already - it's probably just panel furniture and headers that need to be soldered and then calibration
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities