How often does your Vitamin D get tested? Does it stay around that 36 mark and is that why your dosage was just increased? You could probably increase to 2000 IU/day, and see how it affects your levels. Vitamin D is toxic if you take too much of it, though that's rare. It's also a fat soluble vitamin, which means it requires fat of some type to be absorbed. It's best to take Vitamin D with a meal containing fat.
Do you have a condition of some type that requires your ferritin to be tested every 2 months?
If you were on B-12 shots, that indicates you were low/deficient at some point. How long/often did you take the shots? If your level was 1100 when you were on shots, there's a good chance it may not have stayed that high. Personally, I inject B-12 on a weekly basis. Your level should be checked again to make sure it didn't drop back down. As I noted, low/deficient B-12 could be contributing to your tiredness.
Your FT4 was very low before your med change. I'm baffled as to why your doctor lowered your T4 med.
Your results definitely indicate that you're hypo with FT4 at only 20 % of range and FT3 at only 14% of range, in spite of both being within the normal ranges. It's important to remember that "normal" isn't the same as "optimal" and optimal is different for all of us.
When one is on replacement thyroid hormones, TSH is actually irrelevant as it's a pituitary hormone, not a thyroid hormone. At best, it's an indicator of thyroid status, but doesn't correlate well with actual thyroid hormones.
Most of us feel best with Free T4 at about mid range and Free T3 in the upper half to upper third of its range. You have a long a way to go.
It's typical to decrease T4 dosage when adding/increasing T3, but in your case, it doesn't look like your T4 med should have been reduced because your Free T4 is so low in the range - and the blood work was 2 weeks after the med change... Do you have any labs from before the med change?
Do you know the original cause of your hypothyroidism? Hashimoto's is the # 1 cause of hypothyroidism - have you been diagnosed with that? Have you had a thyroid ultrasound to check for nodules?
Many people who suffer with hypothyroidism are also deficient in Vitamins B-12 and D, along with Ferritin, which is the iron storage hormone. Vitamin B-12 deficiency causes the most horrible fatigue you could ever imagine. Both Vitamin B-12 and D are necessary for proper metabolism of thyroid hormones. Iron helps with the conversion of Free T4 to the usable Free T3. If you've been tested for any of these, please post their results and reference ranges so we can see you status. Again, being "in range" isn't optimal and often not good enough for most of us.
Have you talked to an endocrinologist about your results? Hopefully the current dosage (75/12.5) is an increase from what you were taking before. If so then you are on the right path. Be patient over the next six weeks and hopefully you will start to see some improvements. You may not see the full optimum results from the current dosage for up to 2 months. It is a slooow process so hang in there.