So that is approx a 15% difference but as you suggest I would put that down to the B-52 still retaining a nuclear delivery role and the costs associated with maintain that.
Note that the F135, the base engine for the B21, produces some 50 - 70% more dry thrust than the B1's and B2's engines and about double the thrust of the B52's.
I haven’t seen a confirmed report on that yet. Definitely sharing tech from the F135 but perhaps not a direct placement.
A twin-engined B21 would therefore likely be smaller than all three existing strategic bombers but not necessarily by much. Looking at the rumored configurations, it would probably not be able to go supersonic but a high subsonic speed at low level similar to the B1 & B2 is likely. I can see it replacing the B-2 easily but the B-1's conventional role would be a bit difficult.
The B-1 has a role in the conventional space, and found a niche in Afghanistan, but I still think the temptation to replace it will be great. I don’t believe it has ever been supported internally the way the B-52 is.
CAS could be delegated to the F-35, which however lacks a large bombbay and some endurance, or to the B21, which in turn has the payload and range but only low speed. If a B21 was found by enemy fighter jets it would be in deep trouble - it can't run and it can't turn.
I see the B-21 having a self defence capability with AIM-120 or its successor as well as potentially a DEW. Running and turning may not be the best way to defeat for a stealth platform that has some form of self defence.