Low level is much harder to intercept than high level. The higher you go the more visible to everything you become.
Counter-intuitively, high altitude affords the best protection. Radar energy is an inverse square (like any form of light), altitude requires higher energy (read: larger, more expensive) missiles to reach, and high altitude air defense systems are generally the easiest to locate and suppress because they require powerful emitters to operate. So you become less visible to everything that can see you at high altitude, less things can attack you when you are visible, and the things that can attack you can be located by ISR and destroyed beforehand or forced into hiding for ambush-type attacks. Further, energy is stored as altitude, so when you are attacked by high energy missiles you can evasive maneuver much more freely without risking crashing into terrain, and your options for evading interception are much more open in general.
Low level penetration died in Desert Storm
because Jaguar pilots kept coming back with AKM rounds in their helmets.
That said it didn't stop B-2 from being modified for low level penetration (which had as much to do with its mission of hunting TELs in wartime using an LPI attack radar as anything), but B-21 is going to be all high altitude all day like some kind of super slow and super sneaky SR-71.
This wasn't really understood to the extent that it is now back in the '60s and '70s when low level penetration became the norm, because models in the '60s and '70s were about as advanced as Lanchester's square law. Read: hopelessly primitive and incomplete. The synergy of high altitude, high speed (or VLO, speed and stealth are basically equivalents), and advanced ECM is better understood now, which is why CAS is done at 20,000 feet and climbing, why F-35 is better than A-10, and why the intuition of "horizon is the best stealth" is dead since ~1983.
B-1 is built like the proverbial brick outhouse though, so it can manage both things just fine. Just give it an attack radar optimized for maritime/sea search and you have an overpowered supercharged maritime attack aircraft I guess. Since North Point is an island nation, maritime defense is probably a priority. Having an aircraft that can search large areas of sea rapidly (supersonic) for long periods (range) and large warloads (B-1's shtick) would probably be useful.
It's the only way I can think of handwaving a purpose for strategic bombers in the fiction, anyway. It would be egregious extravagance too, since a mixed unit of maritime patrol aircraft and naval attack aircraft al a Bundesmarine Tornado IDS or Australian F-111 would be a superior option, but I suspect that "sperglording" is a substantial reason for the fiction existing in the first place.
I just wonder, if you couldn't make a rotary launcher like what we did for the SRAM, and fill it with Harpoons, or later on, SLAM/SLAM-ER and put it on a B-1 for rapid response strike, though I guess older B-52s would work about as well for that.
That was done on some late-series B-52s in the 1980s, right? Though they only carried Harpoons on the external pylons IIRC, but that method at least could be carried over to the B-1.
It was done in the '90s with B-52H, and the USAF still practices a bit for the sea control mission with Navy P-3s and E-2s. B-52G was tasked with maritime attack and patrol missions from introduction (the -G attack radar had maritime search and attack capability) until the end of WW3 in 1989.
Maritime patrol/attack is the only plausible mission set for a intercontinental bomber without nuclear weapons.
B-1 makes a better platform than a 767, though. It has a smaller signature, except in the infrared, but that's only fixed by flying subsonic, like B-52, or flying so fast that aerodynamic heating melts conventional IR transparent windows, like XB-70 or SR-71.