Well, what you need to do is to come up with a list of six things you know the patchable MUST have. We already have one here: more than one VCO. So, let's start with that as #1, and work out five more.
Now, clearly you seem to be leaning toward something with a proper controller, such as a Moog Grandmother. But is that the right move? Would it make more sense to get something like an Arturia Keystep as both a controller AND sequencer, with that being capable of working with the initial synth and the later expansions? Also, if you eliminate the controller as being part of the synth, it's worth noting that you both widen the field of candidates + lower the price for them.
Next, what functions should the patchable have? Multiple VCOs, check. Filter...? How much "character" do you think is needed there? You're considering adding a Rings later, which is a physical modelling "filter" and therefore capable of some pretty strange transformative behavior, so maybe you can get away with something with a more simplistic filter sound for now and know that, later, you can expand on this.
More functions...modulation sources? Should it have ample LFOs and EGs? You've specced a Maths above, so again, you might go simpler here, either in terms of amount of submodules or in terms of potential complexity. Perhaps the patchable can have the "simple" sources and the more esoteric stuff can go in later?
Do you want this to be a purely subtractive synth? Or more "West Coast"? Or something down the middle such as the Pittsburgh Voltage Lab or the Make Noise 0-Coast? Or something just downright weird? And with that in mind, do you even really want it to patch with 3.5mm cables? Why not bananas, such as the Kilpatrick Phenol, or Dupont patchwires, like on the AE Modular system? After all, you can adapt one connection to the other in many different ways.
Lastly, should you jump right on into Eurorack with a voice module as your first Euro acquisition? Note that I'm NOT suggesting taking something like a Mother 32 and "de-casing" it to put it back into another case, but something like the Intellijel Atlantis, Malekko Manther Growl, Cwejman VM-1S, Eowave Quadrantid Swarm, et al.
So, work along these lines, sort your "must-haves" out into a list, then sift the data according to them. Given the sheer amount of equipment out there these days, there's SOME patchable that'll work on nearly all if not every one of the aspects you need.