Request: Input for multi-purpose frame design for either 700c or 650b

I'm planning my second build. My first was a rando style frame with a fairly low BB (75mm drop), 73 parallel, longish (435mm) stays. It turned out wonderfully, best riding frame I've ever had. For my next frame, I'm planning a path/shopping bike; I have a ton of gravel and hardpack dirt bike paths around me, and grocery/beer/takeout less than a mile from my suburban abode. As such, I'd like to build something that can handle the rougher stuff (this will NOT be a mountain bike) as well as racks to carry stuff home from the store.

My problem is that I can't decide between 700c wheels with clearance for a 35c or so tire, or 650b wheels. What I was thinking was that I could design the frame with a slightly high BB (perhaps 65mm drop) to accommodate smaller wheels, and longer stays (perhaps 440mm) that would handle the 700x35 with a little additional clearance, and it might accommodate both wheel sizes? Am I crazy for thinking this? I would obviously make my life a bit easier by designing for fairly close clearance with 700c wheels, but I'd like to see if I can create enough clearance for some fairly big sneakers.

The next problem would be brakes. Disk brakes would be an obvious solution, but I'd prefer to stick with rim brakes. There's a 19mm difference in diameter between the two rim sizes. Caliper brakes are do-able, but I'd probably need to use 2 sets of calipers since I don't know of any with that amount of vertical adjustment. What I was thinking might work would be to use cantilevers with the PMW cantilever bosses; braze 2 sets of canti bosses in each location (forks and stays), one for each diameter wheel, and only have the mounting studs in the set of bosses that I'm using.

Would this be too weird? Am I making my life far too difficult by trying to accommodate either 650b or 700c? Or am I missing something obvious, like there's a cantilever brake out there with enough vertical adjustability to handle both rim diameters?

Thanks,

Pete

Riv espouses Tektro R556 for 650b conversions (18mm of adjustment).

Pete, I think you should narrow your goals. Pick a way to go and stick with it. You can always do the other option on your next one. You don't want to end up with something weird that doesn't do anything well. That's how I see it.

Keep it simple, imho. A well-designed 650B bike will do all you need and more and I doubt you'll have any regrets.

Versatility is overrated, in my book. Centerpull or cantilever brakes are de rigueur with 650B.