BC, this is somewhat of an anomaly going on, considering the amount of NDT medication you are on. The million dollar question is, how do you feel? FT3 is obviously the primary active form of the thyroid hormone that promotes ATP, metabolism, etc.
There are plenty of cases where people have successful T3 only treatment programs, sometimes due to chronic RT3 issues that stem from excess conversion from T4. With that said, your free T4 is so low it's hard to tell if the RT3 lab accurately reflects your current situation. Again, RT3 is converted from T4, and more so in excess when T4 is at an adequate level, but other factors (T3 pooling, illness, stress, etc.) cause it to convert at higher levels of RT3.
At this point, I don't know if your T4 is just getting a strong demand to hyper convert to T3, or if for whatever reason the T4 portion of your NDT treatment isn't getting into circulation? If you feel fine, your body temperature is stable, and your metabolism & energy reserves are adequate, then maybe drastic changes are not needed. I would review other labs like magnesium, B12, ferritin, double check kidney and liver related labs, and make sure there are no stomach acid issues. Cortisol will also be crucial to ensure that T3 is getting utilized effectively.
You could also look at seeing what a little Synthroid T4 would do, and back off a bit on the NDT and cytomel. Again, if you're feeling perfect then maybe nothing much has to be done. Your iron is good, potassium is mid-range (fine). If other transport related labs are optimal then FT3 should be getting into the body. If you are feeling "hyper" like, over anxious, body temp fluctuations, then you probably need to dig deeper.
