> I honestly don't see anything wrong with using the engine for its UI and "some rendering" kind of sweeps a lot of the complicated 3d light handling under the rug. I think the biggest mistake large engines have made is baking in features as first class citizens instead of those features being part of a standard plugin you could have written yourself from scratch once you reach that stage.

Godot is more or less built that way. The entire node system is an abstraction over the various “servers” and you can even completely forgo it if you want, providing your own MainLoop implementation (instead of the included SceneTree implementation of MainLoop) and then just use the servers directly.

A lot of the editor features are also implemented as plugins. It’s been very easy turning the Godot editor into a custom editor for my game by writing some simple plugins