Co washing, as in washing the scalp not the length, should require only a quarter sized amount. It will not be like shampooing your hair with lots of product/lather. Imo, you are using way too much and that is probably the main cause of the build up. That being said, I cannot use drugstore brands for cowashing (or conditioning) no matter how much or little I use - the ingredients don't agree with me, they build up, and they are never moisturizing enough.

You can kill the build up with a bs wash, be sure to follow with an acidic rinse.
remember, with all advice or suggestions for your hair, YMMV!!
My hair:
2b wavy, medium texture, low porosity, med-high density. Protein finicky.
My routine:
Cleansing: DCNP, every 4-5 days (occasionally use CJ cleansers).
RO: CJSC, occasionally use JCDC
LI: KCKT or CJSL
Gel: KCCC, CJCIAB for 2nd day hair (sometimes third)

Heat is my friend: warm water rinses, steam caps, and diffuse dry. I love CLU cloths and my Denman.