I'm not really convinced you need all that much more stuff on a 600 than on a 200 - I carried less on a 1200 last year (PBP) than I did on some early-ish 200s I did.
PBP is quite a different kettle of fish though. I really wouldn't recommend it, but you could ride PBP carrying pretty much nothing, no tools, no tubes, no pump. I'm sure it'd be possible to buy/borrow pretty much everything during the ride as the controls/mechanics/towns are quite well stocked and in the sticks/night there are so many riders going past you all of the time that you wouldn't have to hassle many before you got someone who could help., or you'll soon happen upon a local who's staying up for the night with water/cake/etc. You may not be popular with some riders for this approach but you'd get what you needed.
Tools/Tubes/etc? Someone will have what is needed.
No food? Someone will have a spare bonk ration. Do this a few times and you can get enough to get you to the next control.
No water? Borrow some, probably even buying a bidon off them, find somewhere to fill them up en route, buy more bidons at next control, etc.
It'd be foolish to attempt it without adequate clothing, but I'm sure you'd still find a way if you were willing to ask lots of people and improvise. This may be taking it too far though.
In 2011 I came across an Italian guy who'd main front light had broken, he only had a little flashy thing as a backup which he wasn't comfortable using to get the 50km or so to the next control. I loaned him my 2nd light (I was using two identical battery powered B&M Ixon IQ lights) and we cobbled together something (with zipties) to mount it to his (recumbent) frame. He finished (and handed my light back with a beer as a thank you). A subsequent light problem for me and then I'd be trying to beg or borrow from someone else.
(Even on a 300 I'll carry a travel toothbrush though, not going to be blagging or sharing one of those!)
Chamois cream becomes more important as the distance increases
If you're unlucky enough that you need it.
When starting out Audaxing I read through lots of threads and decided that slathering down there with Sudocrem was a must and so I duly applied it for my first few 200km Audaxes. Then one morning before another 200 I forgot to put it on and...nothing, I survived perfectly intact. I stopped using it then and have never had to since, even as the distances increased, certainly much less faff and one less thing to carry or worry about on the longer rides.
Everyone is different though, but I do wonder just how many people who use it really couldn't do without chamois cream. I know I'm lucky that I don't.
This goes back to my point earlier. What I take now is much less than I took early on. It certainly depends on the ride (I took more than I'd usually take on my 600 qualifier for PBP as I didn't have much choice but finish that ride, and I was the lanterne rouge). There's nothing wrong with the kitchen sink method, but it's harder work, and it'll take a good few rides to whittle things down to what you really need. Carrying something possibly completely unnecessary may be a good idea if it reduces anxiety levels, but after each ride you need to review your kit and see just how much stuff you took but didn't use, eventually you'll start leaving things out. This process never ends.
I wouldn't recommend leaving Pitlock skewer keys at home though, luckily I was able to patch that tube in situ otherwise that would have been ride over.