Okay, sounds good, best of luck to you. Some vitamins work together, particularly B vitamins work off each other. So you likely have a deficiency in multiple. They sort of wait for the other vitamins to arrive or else they get excreted. They skip the dance if they don't have a date, so to speak.

There's a lot of debate on megadosing B vitamins, including daily vs weekly doses, but generally it is recommended to take a B vitamin complex with whatever B vitamin you are megadosing (don't ever megadose B6!! and tbh I'd recommend that one for sure is taken in a complex so it stays in balance). I regularly megadose B1, biotin, and B12 in addition to the B complex I take, and don't have any allergy issues from it unless I'm also low on vitamin e. Sometimes a side effect of B vitamin megadoses can be increased allergies.

Absorbing B vitamins can be helped with liposomal formulations or methylated vitamins in the case of methylfolate and methylB12 (methylcobalamin).

Sometimes a B vitamin deficiency, especially B1 and B3, can indicate a magnesium deficiency as the two go hand in hand. It can also indicate a general fat soluble vitamin deficiency, as the liver needs fat soluble vitamins to be able to store B vitamins - so taking betacarotene, vitamin e, vitamin k, and vitamin d together can help with storage. These can be obtained naturally through liver, but liver has a type of vitamin A that is not betacarotene but retinol, and retinol can cause health issues in excess so I don't recommend it really for someone who is already vitamin deficient and delicate. However, there is liver jerky (Epic brand) you can buy online or from health stores, it can give hypervitaminosis A from the retinol though, so I again recommend starting with vitamin e, k, d, and betacarotene in lieu of retinol.

While taking 1 vitamin at a time feels like a good way to know for sure which vitamin is the singular culprit, unfortunately it is usually a group of vitamins that work synergistically with each other that become gradually depleted.

Not that you asked or that I for sure know this will help you. I just have celiac and personally have to supplement constantly due to scar tissue in my intestines, so I study vitamins a lot and regularly help people with their deficiencies because I've had so many myself. The way I know how my celiac is doing, is by knowing signs of vitamin deficiencies. I'd be freaking tf out if I had geographic tongue because it means I'm having a bad celiac flare and may need injectable vitamins etc. Geographic tongue is a hallmark vitamin deficiency sign I look for, for that reason. Again, good luck to you, may it all work out!