Why not make the tire clearance 50mm or more?  That is what a lot of people want.  Please!

Where did you end up sourcing the Token headset from? I've had a look online and they don't seem to be particularly easy to find. Is it the TK1556SP model?

• With tires measuring almost exactly 50 mm, I have 4 mm clearance at the chainstays on each side. I expect the tires to stretch a little bit over time, so it's good to know that if they plump out another 2 mm I'll still have adequate clearance. I didn't bother measuring the front, there's easily >6 mm of room on either side, the fork could probably take a 2.1" tire if it's not too knobby.
  
• The total diameter (including tires) of my 650b wheels is about 1 cm less than my 700c wheels; the ~5 mm difference in radius didn't really change handling but did lower the cranks by that much, which is important because I got two or three pedal strikes, even with a decent choice of line so as to avoid the larger rocks. I'll be getting crank boots since my crankarms are carbon. Not really an issue on terrain where you're not trying to navigate through a rock field, but that does make up some of the more fun routes around here so it applies to my use case.
• Probably due to the far fatter and squarer tire profile, the handling at the front was more sedate than with the 700c wheels; it did take a bit of persuasion to rapidly change direction when I was crawling uphill. No real change with how the rear wheel tracked, though, which was nice. Overall the handling is quite stable but I wouldn't classify it as slack, certainly not on the level of a Stigmata or similarly headtube-angled frames. No problems staying upright the few times that the rear wheel broke traction, which fortunately never happened up front.

• The downtube bottle cage mounts are a bit too far up to fit a 750 ml bottle. I used a bottle cage extender (https://www.aliexpress.us/item/3256805494776538.html but the design is generic) to drop the cage about 2 cm down, which works well. I did drink all of that extra water so it's worth it.
• I didn't get any chainslap on the way up even as I was bouncing around, but did get a fair amount on the rutted dirt descent, even in the big ring and lower gears in the back. I'll be putting on another layer or two of clear tape on top of the single layer that's already on the chainstay, just for a bit more cushioning.
• Being able to run a dropper post may be the single biggest advantage this frame has over an actual Canyon Grail. The dropper is pretty much mandatory for the dirt descents I do and I also use it quite a bit to get aero on parts of descents faster than I can spin my 48/11 top gear (i.e. most of any descent). I have the clamping wedge torqued to 7 Nm and the seatpost hasn't slipped at all - I put a ring of silver Sharpie around the base to make sure - which is impressive, considering how fat I am.
• The rear rim already got a few small nicks from rocks flipping up from under the tire, but I've found with my road rims that a tiny bit of clear nail polish hides such marks on glossy carbon very well.
• The wheelbuilding process on these rims was quite nice. LightBicycle AM728 rims, i9 Torch rear hub + i9 Solix front hub (Torch hubs are effectively impossible to find now and purple ones simply no longer exist  ), Sapim CX Ray/Sprint front/rear respectively. Was my first time building with alloy nipples, Sapim Double Square, and they performed no different than the brass ones I've used before. The internal square part is easy to fit an internal tool (https://www.aliexpress.us/item/3256802115683590.html) and rotate, which is nice because it allows a spoke holder to keep the bladed spoke aligned without a spoke key in the way.

I'm using the SRAM T47 BB and even though it's nominally for 85.5 shells, I can confirm it works perfectly fine in the 86.5 mm hubshell of this frame. With RED cranks, the fit is absolutely perfect with a 2.5mm driveside spacer and the preload adjuster turned all the way out. My subcompact chainrings sit a little closer to the frame than stock chainrings would, so with a 2 mm driveside spacer you could get some preload adjustment.