Yep. Even the boring, topology-agnostic 'macro' variables can be good proxies for difficulty: obstacles count, empty squares count, counts per piece type, etc. I don't have a direct source for this but the original echo chess post has a good EDA section (+ feature importance plots) covering the impact of such variables on solvability. At scale, average solvability of random levels might be a half-decent proxy for how the difficulty of individual ones would fluctuate.