A crazy idea
To be taken with a grain of salt, because I'm certainly not a rocket scientist like TB.
There are several designs for semi-buoyant heavy cargo lifters (Sort of up-dated dirigibles) that boast a 500+ ton payload capacity.
I don’t pretend it would be an easy task, and it might not work at all, but what about marrying this concept with a very large lifting body, say something vaguely shaped like the failed Venture Star.
Might it be possible to make a semi-buoyant heavy lift aircraft capable of the speeds and altitude necessary to assist a manned single stage vehicle to orbit?