I would say that your body(liver) is probably using up all available cysteine to make Glutathione which tends to be very deficient in ME/Fibro. Your body is not able to meet it's needs for Glutathione. Despite what GreenEdge claims, TMG is not a good source for Glycine IMO because it can promote over methylation when combined with B12 and folate and can cause ME sufferers to become anxious and on edge which can be very unpleasant IME. The body requires large amounts of Glycine but that cannot be met using TMG. There is research showing how none ME sufferers require higher amounts of Glycine than the body can make and it does go into the amount required by the average person.
A weak link in metabolism: the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis - Glycine is one of the three amino acids needed for Glutathione production and it also occurs every third amino acid in collagen, is such an important amino acid. It's no wonder people show signs of ageing. High amounts of Glycine can help prevent a lot of it if it's GABA raising effect can be countered.
ME sufferers are by no means "average" in their requirement for Glycine. I supplement with large amounts and I try and balance it with Glutamine. I have in the past made Glutamate from Glutamic acid but that was too much trouble TBH. Glycine tends to promote GABA which is calming and in large amounts from taking large amounts of Glycine it can promote sleep. I try hard to prevent that. B6 is important to balance GABA/Glutamate and some sufferers take very large amounts of B6/P-5-P. There are people who warn about taking B6 claiming it can cause problems with the nervous system but some people need very high amounts of it to stay well. It's important for antibody production, neurotransmitter production and elimination of homocysteine among other requirements.
A weak link in metabolism: the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis - this goes some way to explain why some have problems with their necks due to lack of collagen synthesis because the body is using most of the Glycine which occurs every third amino acid in Collagen to make Glutathione the detox and antioxidant enzyme.
Defence, Detox and Elimination are primary tasks for the human body and everything else is not very important. Glutathione is key to all these processes in different areas of the body, used mainly the Liver, Kidneys and Immune system. The kidneys have the 2nd highest requirement for Glutathione which helps protect the kidneys from the highly oxidizing environment in the kidneys. The kidneys are easily damaged from high levels of inflammation such as COVID-19 and septicaemia which can very easily cause kidney failure.
One big problem with NAC, the main source for cysteine supplementation is that it degrades biofilms which can cause problems in ME and make a person feel ill. We need it but it's effects can have very negative consequences.
Despite your magnesium(Mg) claimed to being normal, it probably isn't in reality. We need far more than "normal" levels because it is so essential to the function of so many enzyme systems. It tends to be used up very quickly when there is inflammation. I know when my Mg is low because I get neck pain and Mg always solves that.
You could try Glutathione supplements such as S-Acetyl-L-Glutathione which is the best form to take but it is quite expensive. Liposomal forms are not very good and reduced Glutathione are the worse of all. however with the very high digestive permeability in ME, even Reduced Glutathione should be absorbed straight into the bloodstream in high levels. The S-Acetyl form does seem to have additional benefits because the S-Acetyl group does seem to protect the Glutathione from oxidation and improve the entry and usage into cells. We seem to have problems reactivating Glutathione with Glutathione reductase which might be because it requires NADPH for the reduction to take place.
I have a small amount of it(25g) and take 1/16 tsp, I consumed something that my body had not been exposed to and it caused a dramatic rise in body temperature and my body burned. I took the S-Acetyl-Gl and the heat and inflammation stopped. It worked very well for me.
Suaway sells it@almost €100 for 50g.
govitamins87 on eBay UK sell it @£99.99 for 50g without the any potential problems getting it from within the EU. They also sell very small amounts as low as 2g if you wanted to try it.
Typically, you can spot the missing nutrient by supplementing in a short period, say 2 days. That change could be evident in a number of different processes (sleep, energy, mood, stool etc.)