Did you try these? I’m trying flats this winter instead of spd’s & was looking for a warm footwear solution. My feet really suffer with cold & I blame the constant conduction through the metal and cleat of and spd. It’s just a theory but I’m willing to experiment.
Anyway my search so far has found the Shimano Am41’s and am45’s but I do like a higher boot for the winter.
The products I’ve found are the Palm Gradient Kayak boot The Five Ten Cannoneer and The adidas Hydro pro boot.
The adidas ones are cheaper here if anyone wants to try them.
Link
I also do a lot of kayaking and generally messing about on the mountains so if they are no good for the bike I will get lots of use out of them.