A day later and, indeed, can confirm one of the modules in the MagicMirror was doing a nmap sweep of 192.168.1.0/24 (legitimately, just not clear why the static IP range) confirmed by shutting the module off and temporarily corrected by putting an explicit block rule on LAN > * for 192.168.1.0/24. Still not quite clear what, exactly, loads of requests on :80, :443, ICMP to 192.168.1.0/24 hosts being dumped out on my ISP router ended up doing. Likely, though, the fact that my router was in bridge mode contributed, though I never tried in route/NAT mode, so I can't be sure. Regardless, it's fixed now, but what a nightmare! Thanks @stephenw10 for all the patient help, even if it didn't end up being a PFSense issue in the end! At least it's fairly well documented here so hopefully anyone with similar issues in future will have a reference for other potential problems...
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