I think to me in a sound design synth, interface is just as important as capabilities. Like A4 is plenty capable of going to also sorts of places, especially abusing stuff like parameter slides and feeding one track into the next… but because of the interface I rarely push it in that direction, especially when I have the Pro 2 right next to it where the mod matrix is basically instantaneous to set up all sorts of modulations. But also more minimal stuff will lead you to different interesting sound designs, if the interface is right. Something like a Buchla Easel and a sampler could be pretty genius, mostly because that interface will lead you down new roads.
I think it also really matters what your sound design is for, like if you’re scoring blockbuster films your sound design needs are very different from someone working in music concrete much of the time.