> ZFS needs around 1-5GB of ram per TB of storage

...per unique TB written to datasets with deduplication enabled, of course, not the pool as a whole (even though that ram cost is indeed at the pool level).

there are plenty of useful things to be done with zfs dedupe at practically no cost if you design for it ahead of time. unfortunately for some reason people parachute into every thread where zfs comes up to say you can never ever turn it on, so people are taught not to even try.