Agree with almost all of that. Unless you are targeting some obscure or old platform, I don't even know how you'd justify trying to write a 3D engine from scratch in 2026, except as a fun learning experience.
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.
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.
While the sister comment is down voted, they are not necessarily wrong. AI absolutely makes custom 3d engines easier to make because the math is very well understood.
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 definitely get it to make a custom 3D engine easily. I asked Gemini 3.1 Deepthink (or whatever the Ultra version is called) to make me a 3D engine that ran in CGA on 8086 XT in x86 assembler, and it one shot it perfectly with filled polys, backface culling etc.
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.
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.
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