Much of my fly fishing is in winter, and after years of freezing I finally pulled the trigger on some 5mm boot foot waders from Cabelas/BassPro.
I went through a few pairs of the "spring run" wader (1000 mg thinsulate in the boot) which has a felt sole - pain in the ass in snow but warm.
Last winter I bought a duck hunting 5mm neo boot foot wader from Cabelas with 1600 mg thinsulate in the boot and they were great. No issues whatsoever. Only drawback is that they're rubber sole so wading can be tough. To fix that I bought some of the Patagonia "river crampons," which have the aluminum bars that are unreal on slick river rock.
The 5mm neos definitely get a few sneers from some guys in breathables but around noon when it's 9°, I'm the one laughing as I'm comfortably sticking fish in the face.
Unless your feet just stay warm no matter what, I've found that fishing in a stocking foot wader in winter is the fastest way to nuke your feet. Too much compression underwater which leaves no room for air to circulate around you feet. The boot foot wader boot is too stiff to compress, hence the warm feet.
One other pro tip - I tried all manner of fingerless fishing gloves with the flip over mitt but finally ditched them a few seasons ago and got some Kinco gloves to fish in. They have minimal cloth on the top and a leather palm with leather fingers. A generous helping of Snowseal and they're GTG.