There is a kind of "Multi-BX" from Tetenal which can be used for C-41 as well as E-6. It is avalable in 2 parts for 15L solution, but can be calculated down for smaller batches very easy. I always make 500ml of working solution. I use the same product, but separate working solution for E-6 and C-41.
I use it since 2 years and it works fine.
The problem as stated by the thread starter is that BX2 has a nasty habt of going bad, and this problem will not disappear if he buys 15 l of that stuff. One thing that seems to prolong concentrate shelf life considerably is flooding the bottle with the concentrate in inert gas every time it is opened.
Trying to be the best of whatever I am, even if what I am is no good.