Maybe you should just quit trying to DoS things with ICMP.
Additionally, setting the GW monitoring payload to something other than the default 0 might help to get answers in a more normal way.
Are you experiencing some real issues (as opposed to smoking things with ping)?
Are you suggesting that the pings via ICMP that the Smokeping server sends each minute would be too much for the OPNSense router?
I am seeing OPNSense show a 15-50% packet loss indication on the Dashboard
System - Gateways - Single - Edit - Advanced - Data Length. Set this to something else than 0. If 1 does not help, try 32 or 56.
If you are experiencing packet loss between OPNsense snd the router directly connected to WAN, check for a duplex mismatch of the interfaces involved and check the cabling.