> it literally can't do everything dhcpd can. Notably: custom DHCP options.
I hadn't heard this.
I did a 5min search; I turned up some not-holistic info. Based on those results, I think a portion¹ of the DHCP option set can be set via config file².
For pfSense, I believe DHCP-option-configuration isn't available in the webui yet¹. The community consensus seems to be - don't migrate to Kea at this time.