Proposed powerplant indeed sounds very much like the SR-72 retractable turbine core - folding away to become a pure scramjet.
I think max speed the SR-71 attained was around Mach 3.5 - but Brian Schul reckoned he flew faster on the Libya mission.
More likely your expertise on the X-15 will be relevant - that thing was flying at a mile a second back in the 1960s.
I live in hope of blowing the final bit of my old age pension on a hypersonic passenger flight...