I’ve updated the guide for this year!
Looks like Volcanic Vents is your absolute best bet for getting loot that will be useful for trading - as in, everything in there is self contained, tradeable with itself for chests!
Second to the Vents with at least two individual drop pairs that trade well, comes the Training Fields, Bamboo Falls, Rainsong Jungle, Crystal Pools, Ghostlight Ruins, and the Kelp Beds.
The higher level venues also come with four different encounterable enemies, aka four “different” drop sets, which may or may not help to reward those who choose to farm there. For higher level venues, Crystal Pools will be your realm of choice, with Ghostlight being an acceptable second.
Yes, everything [rarely] drops from everything! These are just the main drop pairings as noted in the Game Database. You will reliably get these, whereas the random extra drops? Just a bonus.