Low B12 is serious in its own right . I'm not sure about the levels for optimal T4 use. From what I've read it seems Iron, Zinc(+ Copper) and selenium [all minerals] are involved in T4 to T3 conversion- so are probably most important and easily missed.
Iron needs Vit C with it to be absorbed well, too. I take B12 by under tongue Methylcolbamin.
Good B12 level has been stressed as important many times on this site along with folate to work with. I have over limit folate, which is odd ,and may mean I'm not utilising B12 well.
It all gets a bit complex and no doubt your head is spinning with the info that piles in- but at least you and others here have a lively interest-unlike too many GPs,looking at their watches half the time.
On that- just build up a background awareness of what are important factors. None is generally more valid than another- as we all differ.
If you are closer to deficiency, as you seem to have been, then that is a general health question which should have been addressed.
Optimum nutritional status enables illness, like thyroid problems, to be better approached.
It's a long haul- so don't expect overnight results.
If you get to see Dr D-P he will no doubt answer many of your very valid concerns but it will take time for T4 to settle in.