Well
Artyoan at some point i gave up on the 6 men version, reached the isle part the fights were won but half the party downed or worse , was far too much struggle . I tried right now the 4 men older version there and no complaints on balancing, absolutely nothing to do with the 6 man version. This time i was using monk , druid, wizard and warlock.The orc fight at level 1 when triggering the chest ? trivial one character was slightly grazed...Incredible the difference between the two versions.
They aren't that different. The synergy between six party members, as well as loads of UB extra feat/subclass options for a campaign that requires UB, means the curve is different but most of that curve up comes later on. Early its mostly a matter of a couple or a few extra enemies. You can refight that same Orc battle over and over again with different initiative rolls on four or six and it will change wildly. I didn't get hit on six. If you do the four in full, go back and try the six again. New party.
I find them MUCH different, in the 4 man version the bandit captain fight is completely doable, a few goons with him in the same room you can manage it much more easily, one control spell in room you can do them one by one mostly and their stats are reasonable. In the 6 man version you doubled the number of enemies , some as i said spawn out of thin air at range, which makes much harder to use control spell, they are using sleeping arrows, and are backed by veterans with three attack by round , one failed save against those arrows and its over already , the veterans will close in very fast and land 3 attacks with advantage. Then the captain is now hp bloated with a weapon preventing party members to heal(and you dont even loot that weapon).
All of that just for 2 exra members ! I think you should change the module description and warn people its for masochistic players with a very optimized party.
No surprise your first version was much liked, the 4 man version is thoroughly enjoyable, the 6 one that's another story...