Stride has a fraction of the features as unity.
Godot is the only real open source competitor, their C# support is spotty. If I can't build to Web it's useless for game jams as no one should be downloading and running random binaries.
A real sandbox solution with actual GPU support is needed.
Writing C# in godot is a bad choice. Use GDScript and directly write c++ as a module. Skip "HD extension" completely. Godots build system is easy enough to use. Just add new classes to the engine using c++ if you don't want to use GDScript. The GDScript workflow is honestly great. Using C# is like the worst of all worlds.
Godot 4 C# web export is coming soon : https://github.com/godotengine/godot/pull/106125
I think WebAssembly could become that sandboxed solution. .NET Blazor WASM is moving away from mono to CoreCLR (just like Unity, with an early preview in late 2026). WASM now has really good SIMD support, native GC, large memory support, and with WebGPU we could finally see some native WASM games running at native speeds.