Welp turns out this whole ordeal had nothing to do with Verizon or my ONT. I did the packet capture and the mystery device sending DHCP signals to the WAN was the pfSense box's own baseboard management controller. My board's BIOS has an option to disable the IPMI function which is supposed to disable BMC networking along with it, but evidently that doesn't work as explained, or is broken. And even though I'd never connected that network interface to anything, the BMC wants a DHCP lease. I logged into the IPMI GUI, set a static config, and I'm now nearing 24 hours of uninterrupted uptime. The 0.0.0.0:67 - 255.255.255.255:68 entries haven't shown up again in packet captures or the firewall logs.
I'm very happy this was smoothed out and thank you @Derelict for the tip to look at the MAC addresses.
@dtruesdale For anyone else in this situation, a few more tips:
You can just set an IPMI address, netmask, and gateway in the BIOS. This is all that's really necessary so you don't actually need expose the BMC to the network.
If you fully configure IPMI and intend to leave it network-accessible, you'll of course want to change the default ADMIN/ADMIN username and password. Through significantly more trial and error than it should have taken, I found that even the latest version of my board's IPMI firmware is so old that it doesn't allow special characters in the user passwords. Despite Supermicro support pages saying that the max pw length is 20 characters, I wasn't able to use more than 16. There's also a handful of service ports that are enabled by default so check those out.
This site has easy instructions to reset the admin pw for if (when) you lock yourself out: http://tcpip.me/2018/06/23/how-to-recover-forgotten-ipmi-credentials-on-pfsense/