Last meeting I mentioned that I have a taproot task to expose a precomputed midstate constructor in the hash accumulator template. Well turns out I did that a few years ago. :))
/// Sets initial state to Hash initialization vector.
constexpr accumulator() NOEXCEPT;
/// Accepts an initial state and count of blocks accumulated by it.
constexpr accumulator(const state_t& state, size_t blocks) NOEXCEPT;