Mileage affects California values, whist some owners use it as their everyday car it seems a lot have it as second car, or just for camping.
This means there’s more low mileage examples than you’d find with a normal car.
Manuals are less desirable in my opinion, not so much with the T5 /T5.1 but fine the T6 is actually quite rare that anyone ordered a manual.
I’ve owned both and I would never go back to a manual California personally, the DSG is excellent and suits the driving style of a camper in my opinion.
The lower prices you mention on Autotrader I think are ex rental ones, can be fine but does put people off.
Colour and spec affect the value of the T6’s too there was a huge number of options to choose from.
The T6.1 came very well spec’ed so less options available, or necessary.
California values are a bit of a hard one to track, so many variables.
I wouldn’t put too much emphasis on WBAC values personally.