Seems like you want something like the Specialized Diverge Evo?

I second the custom TI Waltly approach.

How about a lightbicycle Journey in size large? Gives you effective top tube of 574 and you can add a 60ish mm stem to flat bar.

If you're gonna run smaller 700c tires, then consider a 650B bike frame so your bb drop doesn't get too crazy low if you choose a mtb frame.

For your wheels, just get different end caps to match whatever frame you get. If you go boost, you'll have to dish the rear wheel which is pretty easy or cheap if you pay for it.

There's a few on top-fire but I've never heard of anyone who's got one so far. A couple hub based and a few crank based. Might have to accept gravel tire spacing.

Any thoughts on hub vs bb motors? Hub seems maybe cheaper, more reliable (at least simpler), and probably feels great if lower powered as these seem to be.

https://www.top-fire.com/carbon-road-e-bike-frames_c28

Very interesting, can't find anything myself - got a link?

What do you estimate the maximum rear tire clearance to be? I'm hoping some high volume 2.4's or...god willing, 2.6" could fit.

You best bet is to find one with a pressfit bottom bracket and then install an eccentric bottom bracket. I have yet to find a chinese carbon mtb frame with sliding dropouts.