Everyone told me "use Unity" "the biggest mistake you can make is building your own engine". How well did that advice age? Nobody needs Unity or Unreal. My most recent game (playshadowvane.com) is built on 100% proprietary technology, I built everything in-house down to the physics. Always ignore mainstream game dev advice, they are just trying to sell you products. Build as much as you can in-house, it's not only be…
Now try to hire 20 level designers and 5 engineers who know how to work with your engine/tools. Same applies to most software. It isn't that hard to make a new programming language or a http framework. But once you make one finding other people who know how to use it is a pain in the ass.
Level designers use tools, not really contributing at the game engine level. Long before I built Shadowvane I built a map editor in Three.js that anybody could use. If the game became profitable, in-house tooling would sophisticate and the team would be the ones shaping it. But yes if for some reason my first hire was a level designer, they could add value day 1.