Sled to speed, then light em upDr. Eugen Sanger was working on the same concept in the 1940s. You are correct. I would like to give you an alternate. Why not a sled to carry the craft to power by conventional rockets then its release it using its own power in space
Use a railgun.
Sled to speed, then light em upA long enough magnetic rail might do the trick, although any passengers could get turned into jello at 30 G’s.