Also, think about tooling. Last time I wrote an engine from scratch, the tooling to support it probably took way more time than the engine did.
Most games do not need 90% of the features 3d engines have. A simple geometry batcher with a bog standard rendering equation and simple single directional shadow map will go a very long way. I can confirm AI has been able to one shot this for well over a year.
However, dont expect to churn out nanite overnight
You can realistically get a featured 3d engine+editor up and running in a couple week with AI, working solo. Probably better than what Godot or even Unity gives you. Also AI is very good at editor/tooling stuff, I've even found it getting better at graphics programming stuff, just telling it to 1-shot implementing gpu occlusion culling, ddgi probes, taa, etc. type of features. Also for stuff like animation, I just told my AI "clone Unreal's animation blueprints" and I have a pretty featured animation system now. "clone Unity's particle system" and it 1-shots it in an hour with the runtime and nice editor tools. With the advantage being you can just implement exactly what you need.
(A: there be dragons)