Ascorbic acid--one form of vitamin C--with meals could be considered. It should help you to absorb minerals. Vit C is also involved in production of collagen.
I lost weight while trying to gain it when I had a parasite. A Metametrix stool test reported two parasites that were consistent with my symptoms, especially needing to rush to the toilet at least twice per day on short notice.
Did you ever have a stool test for parasites, candida, and gut bacteria?
Do you have a history of taking Antibiotics ? Maybe your gut ecosystem is messed up. Dr Ruggiero has interesting youtube videos on the subject. He probably is one of the best in the world on that topic.
History of candida/yeast infections?
Ever try probiotics and yogurt?
Do you tolerate whey powder?
Have you ever tried juicing in order to get maximum nutritional benefit and ease of digestion?