I've used seed HHcLC5acQt and it has most every biome on it and what it doesn't is a short boat ride away.

That one appears to have survived the update! I've used that one before and the new map doesn't look much different from the old one.

Added: That's strange, now that I'm double checking these they seem to work. Not sure why they weren't before. I was using cut and paste.

The update appears to have rendered these outdated. Feel free to post new ones when you find them.

Added: Just found one, five biomes, trader, and three bosses on starting island. Good swamps on start. Good number of the new frost caves too. Have to travel a bit for Elder and Bonemass. Have to travel for Mistlands too if that's on your list. wE2ehU3jGG

It's definitely possible to have all 6 on one "island", but for all of them to be usable sizes the islands are really big (long). My seed has at least two islands with all 6, one of them stretches from spawn to damn near Ashlands though. The other has everything but meadows is small. Just gotta keep looking till ya find the right seed.

Its pretty common to get meadow, forest, mountain, plains and swamp within a 5 min walk of a central spot. Typically, out of necessity, one or two of those will be rather small; one place like that the swamp part I could jump across the whole biome but it would spawn leeches and other stuff on occasion. And there are on occasion crazy long (almost 1/3 of the map runs) land masses that have it all, but it may as well not because the sheer distances are too far to walk -- a portal on the same island or on 6 islands is the same in the end and boating is much faster and easier in general (noobily mishaps aside).

The goldy lox zones are hard to find, you just have to keep exploring!

My southern base has plains, BF, ashlands, mountain, swamp. It looks wierd on the map

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.

likely pretty rare, I have one that has the main 5 all contiguous, and from either end you could swim a narrow channel to reach either of the other two. You can walk to all the bosses and the trader from world spawn.
At 3 and 9 o'clock on the map, in the seed reader just hit random a few dozen times. You may find that and others that are intriguing. It is how I found my Mtn Ring Trader/Citadel Seed.

This was my first one pangea like seed , surely will help you a lot :
fMv2IEMhRA

Here is a seed with 5 biomes and the trader on the starting spawn island: EeaJGT0Xsn

not certain what your definition of perfect is. rCDirPsjcv has a nice size meadows island @ 210, 1680 North-Northeast of spawn. decent placement of crafting benches disallows events from spawning any creatures. for plains veggies I just wall off a good size plot and make a portal to it. so far this has been my overall favorite seed.

I had an island bordering Ashlands so it had 6 biomes. But yeah it seems hard to find meadows amongst the outer islands with multiple biomes