Your guess's right. So far Volksauto has gave me 4 custom limb parts (all limbs), plus recurring merch hoodies or shirts, other than these I haven't found anything new.
I've reached the top position, but to my experience it shouldn't affect volksauto constract spawn, since Im able to get the contract, just after a few days of contract being available from my new save (o w o ).
So… yeh, maybe it is all about rng…
(o w q )