Heck, Saturn V was even decent for manned loads. Not an engineer, don't know how much Saturn V could be improved if we dropped the manned requirement.
And whatever happened to magnestic acceleration? We got to over 1000G without accelerating fuel, back around 1980. It wouldn't have to reach orbit - there are some very interesting hybrid designs.
Just avoiding vertical launch is a huge advantage. The ability to bring the launcher to 40kfeet and a few hundred mph saves a huge amount of fuel.
One idea is to literally tow it into the air. I understand there was a test with an F-106 towed into the air by a C-141 at Edwards AFB that represented towing a high speed vehicle up to altitude. Worked fine.
I've got to wonder what the performance would be of a scramjet/rocket vehicle launched from the top of an XB-70 class aircraft like the drone that launched from the top of the SR-71. It seems hard to believe, but the XB-70 held the absolute weight lifting record in the days before the 747 and C-5. I'm sure it could have lifted a hefty vehicle on it's back. Don't know how fast it could have accelerated it, although it was designed to cruise at Mach 3+. I'm not sure if you can light a scramjet at Mach 3, but I think there are some combo scramjet/ramjet designs out there.