EROB The recommended level for Vit D is 100-150nmol/L. Over 75 is replete. So at 85 you aren't too bad, you should be able to boost that by getting out in the sun with skin uncovered (no sunscreen) for maybe half an hour a day. You could probably do with a maintenance dose during the winter, say September/October to March/April of maybe 1000-2000iu D3 daily. If you do supplement you'd need K2-MK7 as well, you can get combo D3/K2 supplements which might be easier.
Were you tested for B12? If so was folate also tested? As you're taking B12 are you also taking a B Complex to balance all the B vitamins?
What is in your 'thyroid complex vitamin' ? Does it contain iodine/kelp? If so we shouldn't take iodine unless we have been tested deficient.
T3 - 3.5
T4 - 18.6
TSH - 0.79
Do you have the reference ranges for these?
TSH is obviously at the lower end of it's range which is good, most of us Hypos feel best when TSH is 1 or below.
FT3 is very possibly low in range, FT4 I have no idea whether that is near the top or not. What is obvious though is that you are not converting T4 to T3 very well. Good conversion takes place when the FT4:FT3 ratio is 4:1 or less. Yours is 18.6 : 3.5 = 5.31 : 1 .
Selenium can help with conversion, as can optimal levels of vitamins and minerals so it's a good idea to get them all tested as mentioned.
If all the building blocks are in place and your conversion is still poor, then adding T3 to Levo or changing over to natural dessicated thyroid (NDT) is worth considering.
Stress comes in many forms (chronic health conditions, poor sleep, as well as the more obvious forms) and stress can affect adrenals. Adrenals and thyroid are linked. You might want to consider a 24 hour saliva adrenal stress test to see what's going on and whether you need any specific help, but lots of Vit C and a B Complex help support adrenals generally.