Hybrid engines
If it was me, I would design a NERVA-type reactor to produce high thrust by piping hydrogen through it. Then for cruise phase, damp the reactor down and circulate helium through both the reactor and a turbine-generator pack to make power for a high-ISP, low-thrust electric drive.
Of course, that's just an engineer talking. Don't know that much physics, and I don't know if you can design a reactor to have that wide a power range.