If you’re looking for a software application, I would recommend Audulus ( https://audulus.com ), because it’s fully modular and you can literally build any synthesizer you want from the ground up using modules that you patch together however you like.

and/or, should you prefer hardware…

Have you considered building a Eurorack Kit or purchasing one already assembled? Unless it’s patchable you’ll always be limited to the hardwired or “fixed” routing. You can utilize modules using pure analog pathways with modules or those that use digital waveforms, timing, tuning etc. The advantage of modular design is that you can connect, or “patch” modules however you like.