You could programme it to fly over multiple enemy targets and release a warhead over each one and then overload the reactor and detonate the last warhead inside the missile... why bother with SLBMs?
They are really big and heavy and not invulnerable to ABM systems except with sophisticated warheads...
Perhaps keep a mix... one third Boreis with SLBMs, one third with unlimited range cruise missiles, and one third with conventional cruise missiles that can be used in conventional conflicts...