Your firewall is very much the last thing that you want compromised. Please put cups on a different host.
I have a Brother color laser printer that does not support iOS AirPrint.. I had looked into running a CUPS server on my OPNSense box..
I had also tried on an RPI3.. In the end, the RPI3 was not powerful enough to render the postscript necessary, and what I ended up doing was setting up a Debian VM on my Windows machine that is always powered on.
I have a Brother color laser printer that does not support iOS AirPrint.. I had looked into running a CUPS server on my OPNSense box.. I had also tried on an RPI3.. In the end, the RPI3 was not powerful enough to render the postscript necessary, and what I ended up doing was setting up a Debian VM on my Windows machine that is always powered on. Takes up a tiny footprint, and worked great.