I think what you were looking for was FixedSizeArray2<Pointer<RgbKey>>.
But in this case, with a dynamic sized array, it probably won’t help much.

Could do

[FieldOffset(0x4)] public RgbKey* KeyPointer;

public Span<RgbKey> Keys => new Span(KeyPointer, KeyCount);

with looking at how the GetKey function works

I think what you were looking for was FixedSizeArray2<Pointer<RgbKey>>. But in this case, with a dynamic sized array, it probably won’t help much.

Originally tried this with a FixedSizeArray1, but it throws a IndexOutOfRangeException beyond the first element.

Could do

[FieldOffset(0x4)] public RgbKey* KeyPointer;

public Span<RgbKey> Keys => new Span(KeyPointer, KeyCount);

with looking at how the GetKey function works

There isn't a pointer here, it's an inline array with a dynamic size.

this will export the unions as

union Value {
  uint64_t Value;
  uint32_t Id;
  uint32_t Index;
}

all which are at field offset 0