For indexes, I completely agree! If I were to add support for larger amount of keys, I probably would introduce two versions of the data structure, with 16-bit

For indexes, I completely agree! If I were to add support for larger amount of keys, I probably would introduce two versions of the data structure, with 16-bit and 32-bit indexing. And, maybe, 8-bit indexing for tiny amounts of keys. But that would definitely complicate the design, and should be done only when there's a real need. Every such decision is a trade-off; I think yours is fine.