T3-only is rarely a satisfactory plan to treat low thyroid. T4 stores are every bit as important as the FT3 level. The brain (and I
think muscles) need a supply of T4 to convert on site. It (they) can't make use of T3 directly. So yes... I agree you need a T4 supplement. You're a good candidate for porcine thyroid med, since you need both hormones.
Suggest to your MD that trying to make do with FT4 of .8 when you need it higher would be like a shoe salesman insisting on selling him a size 10 shoe when his foot measures 12. Ask how he'd like that.