What do you mean by standalone renderers? There's Forge [0], which was used for Starfield. Also, there's nothing stopping you from taking a popular engine and using it as a renderer only. Remaster of Oblivion is running the original game code underneath and using Unreal Engine 5 for rendering. I assume Diablo 2 remaster did something similar because you can seamlessly switch between old and new graphics.

[0] https://theforge.dev/products/the-forge/

I mean yes you can do that, but it’s pretty hard since the engine expects you to work within its framework. Not saying it’s impossible of course, just annoying. Also, if I’m not mistaken, the forge is just a cross platform graphics wrapper? You still need to write a GLTF renderer and all that yourself if I’m not mistaken.