Bullet VS OpenSceneGraph

For typical game physics engines... not that much. Math libraries like Eigen or Blaze use lots of template metaprogramming techniques under the hood that can help when you're doing large batched matrix multiplications (since it can remove temporary allocations at compile-time and can also fuse operations efficiently, as well as applying various SIMD optimizations), but it doesn't really help when you need lots of small operations (with mat3 / mat4 / vec3 / quat / etc.). Typical game physics engines tend to use iterative algorithms for their solvers (Gauss-Seidel, PBD, etc...) instead of batched "matrix"-oriented ones, so you'll get less benefits out of Eigen / Blaze compared to what you typically see in deep learning / scientific computing workloads.

The codebases I've seen in many game physics engines seem to all roll their own math libraries for these stuff, or even just use SIMD (SSE / AVX) intrinsics directly. Examples: PhysX (https://github.com/NVIDIA-Omniverse/PhysX), Box2D (https://github.com/erincatto/box2d), Bullet (https://github.com/bulletphysics/bullet3)...

I suspect most C++ physics libraries like Box2D (https://github.com/erincatto/box2d) or Bullet3 (https://github.com/bulletphysics/bullet3) could really benefit a lot from SIMD.

No, they replaced Bullet with Jolt. That is considerably more than "some adjustment", regardless of what you think of the result.

Vulkan graphics via Intel GVK, and physics via Bullet

The first two have a bunch of great examples, and I’m tying them together by refactoring some of the THREE examples to fit the ECS paradigm defined in AFrame. then upping the ante by adding physics using AMMO, which is more challenging since it’s only a partial implementation of Bullet, and already poorly documented (yet popular) physics engine.

Plus, SM uses a system called bullet physics, I imagine this would be rather complex to rework into a modern engine such as Unreal or Unity (after all, the majority of performance issues come from the physics engine rather than the graphics engine)

The physics in HDT-SMP are actually being calculated outside of Skyrim's engine with Bullet, an open-source physics engine. So this isn't some limitation of Skyrim's engine.

I started in 2001 with OpenSceneGraph which made extensive use of intrusive pointers. This was 10 years before C++11 note. That codebase continues to be relevant and as performing as ever.

browse github and try to contribute. I started with OpenSceneGraph

I have been using OpenSceneGraph for over 20 years. It is used in the military a lot and I myself have seen it used to simulate war scenarios with realtime data aboard of a command-and-control center of a battleship.

Does http://www.openscenegraph.org/ fit?

Speed Dreams: It is a fork of TORCS, and was born as the need to include many more functions to the latter. In Speed Dreams the menus would be completely redesigned, adding many more options and making them much more intuitive; the game acquired dynamic time, improvements in reflections, career mode, a new simulation mode and multi threading. Over time many more options were implemented, such as local multiplayer mode, Force Feedback, and of course more and better cars and tracks, a new and more powerfull graphic engine (OpenSceneGraph) among other things, making it a much more complete game than its predecessor. They recently they have presented version 2.2.3. You can also install it in AppImage and Flatpak format.

Here is an example: https://github.com/openscenegraph/OpenSceneGraph/blob/master/examples/osgsimplegl3/osgsimplegl3.cpp

openscenegraph (mainly focuses on graphics, used by openmw for example)

Is not little, is very complex open source racing simulator. There are a lot of cars, categories, tracks. The physics has a lot of work . About the graphics, the devs are working on change the graphic engine to OpenSceneGraph, that actually works, but needs much more polishing. You can see his quality on this video: https://youtu.be/4daTgcxEWRg

I love Godot for a complete game engine. For just the renderer you could consider http://www.openscenegraph.org