Multivitamins have minimal amounts of most nutrients in ratios that may or may not be right for the patient taking them. It is an erroneous assumption to think that they would be sufficient for any sick patient, especially someone ill with ME/CFS.
There are numerous reports of people adding thiamine to their regimen and it being a real game-changer for them.
That's a pretty low dose. I'm taking 750 mg of Benfotiamine or else I test low in thiamine.
This describes what thiamine does, including reducing fatigue:
https://selfhacked.com/blog/thiamine/
Exactly how did you do that and for how long? Reducing them in your diet is one thing, but that just stops them from adding to the deposits you already have. Typically, if you have an oxalate problem You will have oxalates deposited in places throughout your body, including your eyes, heart, kidneys, joints, mitochondria, and everywhere else. It typically takes a long-term effort, including taking potassium, magnesium, or calcium citrate prior to each meal, a good deal of vitamin B6 in the form of P5P, and a low oxalate diet, for 1 to 7 years. The idea is to reduce them being deposited, and then to lower the gradient so that they start to come out of where they are stored. Oxalates can cause all kinds of mischief while you have them. They are a toxin, but they do not use the normal detox pathways, this is how you have to detoxify from them.
This is more than unfortunate. It is directly damaging to your body, and put stress on your ability to detoxify, which sounds like it is already stressed. This would greatly increase your need for detoxifying nutrients.