The survivability of B-21 is in context of it conducting much closer in raids.
The H-20 we are talking about would be conducting initial high end missions at distances of 2000-3000km away from its target.
As for planform, what you've depicted is somewhat close to what I also imagine but I think it is a little bit too extreme. Something like below (with approximate characteristics), with B-2 for scale.
It's just for illustrative purposes of something I have in mind, the exact detailed numbers aren't so important (for a 10mg or even 15m length weapon bay you could probably make do with a smaller aircraft, like 30m length, 24m span)
View attachment 158950
A 30m length, 24m span planform alongside for comparison, while keeping the same IWB length options.
Needless to say it would be a bit tougher to do in such a planform, but it also depends on the depth of the main bay, which could still make it fairly viable to mount four engines and the requisite air intakes

What would be the rough range and payload of this aircraft?
I haven't really thought that far ahead, but for the 35m long version it would be no less than a B-2, probably a bit more in terms of weight.
To be honest in many respects it wouldn't be too dissimilar in scale/size to the low altitude penetrator concept (an AoA related to the ATB that led to B-2), except this would be oriented for high altitude performance (dorsal intake, no ventral fins or any fins for that matter), and focusing on IWB length rather than bay depth, and of course also using a much more modern planform.
