My current seed has 5 biomes on a large central island. The trader isn't terribly far from the spawn point. It has two massive mountains with several smaller ones, a massive swamp (tons of crypts that I'm still working through), and a large plains zone. All the bosses are on the island, too. WBFzViM6sG
There is a small island to the north (-484, 5307) that technically has the 6 biomes, but given how small it is (mistlands is just a small sliver), it probably wouldn't be worthwhile.