>I am not aware of an easy way to load (managed) mods as DLLs to IL2CPP-compiled game. I am thinking about `Assembly.LoadFrom("Mod.dll")`.

Ah, I was thinking native DLLs (which is what we're using on a project I'm working on). I think you're right that it's impossible for an IL2CPP-built player to interoperate with a managed (Mono) DLL.

>If you have some suggestions [re: serialization], I'd be interested to see what options are out there for C#.

We wrote a custom, garbage-free JSON serializer/deserializer that uses a fluent API style. We also explored a custom codegen solution (similar to FlatBuffers or protobuf) but abandoned it because the expected perf (and ergonomic) benefits would have been minor. The trickiest part with Unity codegen is generating code that creates little to no garbage.