You really had some bad luck with the crypts. The first swamp I found was tiny and had no crypts. The next one was quite large and had five crypts. Then I found a massive swamp and it about eight crypts. Sure it's still a chore since you have to mine and haul all that iron back, but it didn't take me that long really to amass enough iron to make all the things I needed. You can keep mining and hauling the ore back to the boat. Just mine one crypt, then sail back with a nice haul to your base. Smelt some iron, build a long boat, and sail back. Now you have huge storage and can park your boat and mine the hell out of the other crypts for several game days, then sail back with an enormous load.