So... only arctics and your surprised you get cold?
And blaming hypothyroidism for it??
COME ON!
Get a proper undergarment to layer with the arctics.
The weazle is used by one of the finns doing the longest, deepest coldest cavedives... Should be good enough...
I use the kwark. I layer it with 1 or 2 layers of thin merino wool, and a heating vest and stay warm for 1,5-2hrs in 4c water.
My buddies layer the arctic with the Santi BZ 400. and 12+ (55) degrees is still quite some months untill we see...
At the moment, we have 7C and I dive.
Ursuit Soft Dura (About the same thing as your one), Kwark, heating vest (didn't turn on last dive) 1 layer of merino wool. Proper dry gloves with felted merino inner gloves and a 10mm Waterproof Hood.
Get a proper hood, merino wicking undergarment, a proper undergarment on the top and some really nice wool gloves.