Random thoughts here:
My hair does not like deva products and I think it's the chlorides - I have the exact same issue with suave and that is the only common ingredient that I could see. Point being, it could be something you aren't even realizing could be a problem (chlorides? Really? Lol).
My first thought, like another poster, was protein until I read you tried protein free. When you went protein-free, are you positive you had no amino acids, hydrolyzed anything, etc? Also, did you eliminate avg and coconut oil? Those can result in protein-like issues for some people and should be eliminated as well.
Are you low-po? If so, do you utilize any techniques to open cuticles prior to conditioning?
Last, humidity/dew points and humectants can cause issues. Have you looked into that aspect at all?
Again, just random thoughts on things you may or may not have tried or things often missed. Sorry you are frustrated - it took me 15 months before I finally had more good days than bad and that was after finding my hair properties which you have done - I know I would be very frustrated if I still wasn't seeing any improvement.
ETA: I also had to resign myself to pricier products - the cheap options simply don't work for my hair. And even with pricey ones, it's picky!! Maybe yours is, too. *sigh*
remember, with all advice or suggestions for your hair, YMMV!!
2b wavy, medium texture, low porosity, med-high density. Protein finicky.
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.