FWIW, experience from the great white north.
Down to 20, I wear insulated Schnees pac boots. Below 20, I wear LaCrosse Ice Kings. They are a very fine, yet economical extreme-weather boot and can be had for under $100 on sale. Below -10, I sometimes add chemical hand warmers.
You'll see folks recommend things like bunny boots. Wrong tool for the job. They aren't warmer, just waterproof. In a tree, your feet will be dry.
I think Schnees would meet your needs in IL and they are also a good walking boot. Ice Kings are like putting pillows on your feet and you should not get cold above zero, even if you have a tendency to get cold feet.