Might have been suggested before... but couldn't find any - mostly just topics exist about balancing mine support costs.
Could we have a kind of a mine support that requires no metal?
It could be some wooden support that requires more rope or just more wood (or bone glue, for example).
Or some kind of a brick (or stone) vault or arch - without metal required.
It may have e.g. 6 tile radius, as opposed to the (currently) lowest 9 tile radius one which, however, requires metal.