There are several possibilities, most of which have already been raised.
Totally agree that the dry brushing thing is ... Well it's kinda like walking through a wind tunnel with the hope of having shiny, healthy curls on the way out.
To the issue of hard water: Thing is, a typical water filter won't filter out magnesium sulfate and calcium carbonate (the usual dry hair culprits). For that you either have to get a water softening system (costs hundreds of $$$ or more), or you need to use an occasional chelating shampoo to get rid of the associated build up from these mineral compounds. ... Guess which one I chose?
As to dryness, yeah, silicones could certainly be locking moisture out of your hair and causing build-up. Often a commercial clarifying shampoo (over one or more wash cycles) will remove it. Then you have to limit or avoid cones unless you intend to do a weekly or bimonthly clarifying shampoo.
Too much protein will cause curls to lose elasticity, and to become stiff, coarse and dry. For some, the hair actually breaks off in places. It's possible you need to limit products with protein.
You might want to try a DC (deep conditioner) once every week or two. GVP is very hydrating, so you could gently work that through your WET hair (never dry) and then place a plastic cap or some suran wrap on it, then wrap your head in a hand towel or reg towel and let it sit for about 15-30 min.s like that. The body heat from your head is what helps the condish deeply penetrate your hair. Rinse it all out. That should help w/your ongoing moisture retention issue.