For a pure C# engine maybe check out https://www.stride3d.net. It's using Bepu for physics.

It sounds like a mess, though comments about Godot are often vaguely positive. I appreciate you posting your experience about a limitation you’re running up against.

I can think of only one popular game of note that was made Godot, Slay the Spire 2. I’d love for more titles to be released to prove the engine can be competitive.

Jolt is essentially the same physics engine that was used in the Horizon games, the performance there should be able to get quite good.

If you're making any kind of even slightly ambitious 3d game you probably need to be using gdextensions (ie writing custom engine-extensions in C++). I'll add that the engine source is very accessible. It's deliberately pretty bare C++, so if you have a decent amount of C# / C experience you should be able to find your way around it pretty easily.

Unity is a bit better when it comes to performance foot-guns and messiness (and in some ways much worse). Unreal seems to be pretty frictionless for getting something working that looks impressive, but if you're not very experienced performance is going to be a big issue there too (there seem to be very few games made in Unreal that run well on mediocre / older hardware).

Godot's a mess in the sense that it is trying to expand in every direction at once right now. I think it'll be in a pretty different state in 2-3 years.

Is there actually a huge performance gap between Godot and Unity/Unreal? I've never personally made a game but I have only seen small-medium games in Godot. I would have thought the rendering would be where the slack is tightened first as its the most visible part.

Have you tried using the profiler? Where is the bottleneck?

Sometimes it is possible to pinpoint problems if not fix them. In my limited experience the Godot source is quite accessible and straightforward despite the complications of multiplatform builds.