I've finally cracked what I believe is the last issue on the sender side. I've got the watchdog enabled in the sender but apparently the Arduino Pro Mini bootloaders don't clear the watchdog timer when they reboot so the atmega sits in a rebooting loop flashing the green LED very quickly.

I've replaced the stock bootloader with an Optiboot version which appears to be what is shipped with the current IDE's, but you still have to hardwire an ISP of some kind to do the update since the Pro Mini's lack an ISP header.

The I2C temp/humidity sensor seems significantly more accurate than the old DHT22 which is also noce, even if it does bump up the price I think it's worth it long term