Remember that the B-21 still has a nuclear role. I know nobody wants to think about it, but they're intended for penetrating the Russian (or Chinese) air defenses and dropping nukes. So as a bare minimum, we need 2x the number of B-2s produced, increased by the flightworthiness % of both B-2s and B-21s, just to cover the existing SIOP demands. Replacing B-1s with B-21s, we need 3x the number of Bones, though that's for conventional bombing only. Replacing B-52s with B-21s, we need roughly 2.5x the number of Buffs, and even then I don't think the B-21's bay can hold ALCMs.
So make more external pylons? Replace the belly skin or whatever they did to make the Bones unable to use the externals, replace the wiring with the current smart bomb data bus. While they're in depot doing all that, stuff a quartet of F119s in there instead of F101s, have an extra 5000lbs thrust per engine (F-135s are probably too much oomph).
Because the Bones are certified as conventional-only, START no longer applies to them.
And yes, launch light on fuel then tank if you're playing with all the externals.
Because the USAF is going to need every single LRASM shooter it can get if a fight with the PRC kicks off.
