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
A few more recent non-UE5 custom in-house engines that have demonstrated very impressive results:
* Spiderman (1/2/Miles Morales) and the PS5 Rachet and Clank, by Insomniac on their in-house engine.
* Astro’s Playroom and Astrobot by Team Asobi uses an in-house engine and act as the de-facto hardware showcases for the PS5
* Horizon Zero Dawn/Horizon Forbidden West by Guerilla Games, which shares the custom Decima engine with Death Stranding by Kojima Productions
* The upcoming GTA6 by Rockstar. Not out yet, but the trailers supposedly were captured on a base PS5 and visually look astounding.
* Doom 2016/Doom Eternal/Doom Dark Ages by id, running of course on the latest versions of idTech.
* The Forza racing games continue to use the in-house ForzaTech engine, and Forza Horizon 5 looks amazing visually
* The Call of Duty series continues to use the in-house IW Engine
* The Battlefield series and EA’s various sports games continue to use the in-house Frostbite engine
* Ubisoft’s various games continue to use in-house engines. AFAIK they actually have three major in-house engines: Dunia, Snowdrop, and Anvil.
* Tiny Glade is a small indie game built by a two-person team running on a completely custom engine that despite the team being tiny happens to have possibly the most advanced light transport in any shipping game engine today
UE5 is gigantic today of course, but in-house engines are still alive and well today and constantly producing amazing results.
There is only UE5.