Ah yes, I missed that. You are quite right that this is a significant barrier; making that library multi-band would be significant effort from my brief skim.
There are newer and/or more complete stacks that fit on the 328p (maybe more, I’m not sure if BasicMac fits) but that’s quite a voyage from where you are (moving stacks isn’t much fun). I’d suggest moving to 32bit to make it easier but just looking at 328p power usage (~4mA) vs STM32 (~13mA, 3x!) shows me more about your design constraints 
Great work, again, it’s really inspiring to see how much you’ve squeezed out of it all and how permanently self-powered indoor devices can be in our future homes and workplaces. I’d never imagined that was possible.