[lifthrasiir]

That's a better reasoning in general (see my other comment for the GP's scheme), but you can do much better.

First, you should encode the number of total pieces (and their general compositions) in the beginning, which can greatly shrink all subsequent fields. For example you are using a repeating square index to denote the end of piece list, but you have at most 15 such pieces unless there is a chess variant where new piece is created out of nothing, so you can just encode that as 4 bits instead of a 6-bit separator.

Second, use a fractional number of bits per each field. Each piece occupies one square and prohibits some more squares, and those squares should not be available for encoding. So you need an arithmetic or equivalent coding for the maximal efficiency. A careful ordering can then even eliminate much more squares from further encoding. I think you've essentially done this for pawns, but you can do much more in general.