I am hoping this fairly ancient -5100 appliance holds on. It's been through a lot including several dead cable modems. However, physical intervention has to wait until my next in-person visit. I only have non-technical people on site (as indicated by the hard power off recovery).
The site is in an area with buried cable and a high water table which is a recipe for disaster (eats a cable modem every 18 months) and explains the starlink back-up. We use tailscale so remote access is fairly tolerant of CGNAT and our windows DC / local users can phone home and network admin can remote in. When the comcast circuit is down I lose remote admin via the fqdn/public IP which is stressful as I'm reliant on tailscale coming up using starlink.
As a first order trouble shooting step I'm stepping up a local VM as a tailscale client to give myself a back door if the netgate box becomes unreachable again and potentially to automatically attempt a pfsense reboot in the event of a sustained loss of connectivity.
I will experiment with turning off gateway monitoring after that and watch for physical events. Next time I'm on site I'm going to insert a fiber media converter between the cable modem and the netgate (or it's successor if I can get the budget for it) to remove physical plug events from cable modem reboots and risk of electrical shocks on the wan port.