I think the frame is relatively simple. If you have toured and are of technical mind set, moderately, then you should have a good idea on fit, and you should have enough experience with various tubes to know what range you get solid performance out of. I think the main thing beyond that is to accept that you are going to have to build a couple of frames to get it right where you want it. It's a bit like marriage. If you are lucky the first marriage may be perfect. But if not people tend to rebound, decide the one feature they need to go overboard on is the one missing in their initial go around. It can take two shots to get it right. I have been down that path so many times with various projects. Though not marriage...

I am not a big believer in the integrated rack. I don't see the point. If the idea is that one needs to have a stronger attachment point, there is no reason why that has to be done at the cost of reducing the versatility and options for a rack. The same line of thinking would suggest that the drops should be welded to the axles. There isn't anything wrong with properly sized bolts. But the attachment points should be larger. the current eyelets were really designed for fenders. I haven't really seen a successful integration of the rack in a way that allows the rear end and the rack to be made more efficiently, so they share common parts, save weight. On a rear suspension rack, I think there are some better options.

One thing that does work really well/differently, is to integrate the rack into the panniers. Solid shell panniers that do not require a rack are very light and potentially far more aero. I made a set out of 1/8 inch ply and glass for my mother's city bike 10-15 years ago, and they were really useful and light. The problems are that you are stuck without a rack; they don't ship well; They don't deform to accept that little extra piece of gear; you can't detach the bags; etc... But for just riding with they are excellent, which is why they are so popular on motorcycles, at least the hard shell part. Many of these problems can be worked with, but every rider will have their own features list so best of luck to the commercial maker.