It is a devil trying to please hair and scalp at the same time I do feel for you, I have atopic eczema and contact dermatitis. Luckily dormant at present, thanks in large part to changing my haircare routine and losing the sulphate surfactants but I had a mild case of CD the other day.
I'm in the UK so perhaps not the most helpful on products but I will try. I use Garnier Nutrisse Radiant Blondes for colour and that is ammonia and sulphate free, just bought Garnier Nutrisse British Blondes (in error) and that is also sulphate free but contains silicones and ammonia so is not ideal. You might have different variants where you are but Nutrisse would be worth checking out the ingredients, it's quite a large range so hopefully you would find a product to suit. Certainly claim to work on greys, and do something to my 'silvers', they take up and hold the semi permanent dye I use after (pink-red) really well and hold it for many weeks but mine are probably only average resistance TBH.
When you say get your omegas you are taking in long chain omega-3s fro oily fish not omega-6s (unless GLA) yes? It's the balance that it critical not just the total amount, most of the omega-6s (except GLA) are pro inflammatory. Definitely speak to the in house nutritionists, ideal resource on your doorstep.
With respect if you have frizz and tangling you have cuticle damage, it just might not be evenly spread all over your head. Shine you will have if you are using silicones, that is what they do best and they are in virtually everything commercial these days. AFAIK
loss of body/ curl tends to come with severe damage to the core protein structure of hair (esp. high heat or chemical damage) more than damage to the protective external layers (esp. mechanical damage).
Kinky Curly Knot Today is supposed to be amazing for slip and detangling and is virtually oil free, you also get great slip and detangling from products containing one of the behentrimoniums.
Kinky-Curly Knot Today - CurlMart
You might try a product that also contains coconut oil: research suggests it can reduce protein loss and porosity from damage including colour treating, and is low in oleic acid (rich in lauric acid which can diffuse across the cuticle). Some find the anti microbial properties of coconut oil beneficial in acne and SD, some do find it clogging so I would only get sample sizes.
Conditioners with a behentrimonium and coconut oil include Komaza Care coconut curl milk, Bee Mine Deja's hair milk and Afroveda coconut moisture milk. Thinking about it you could even e-mail Komaza Care and ask for recommendations from their ranges, they seem to be science oriented and I've heard positive things about their customer service. They actually offer detailed hair and scalp analysis (not suggesting you pay for this, just pick their brains!). I've only used a couple of their products but I do like them and they also get good reviews.
Komaza Care - The Complete Hair Solution