That's the argument for general-purpose rendering engines. These offer an API comparable to three.js. The API at that level is much easier to deal with than the API at the Vulkan level. Usually, about a page of code will put a glTF model on the screen.
But fast, general purpose rendering engines are hard. Not impossible, just hard. In open source land, we have about a half dozen examples of My First Rendering Engine, and they mostly work. But they don't scale.
Do open source engines count in your custom engine list? I could see that going either way: yes, because one can modify it to your heart's content, or no because it was built by someone else and not to the exact trade-offs you have in mind
