I spoke too soon. It got a lease, but lost it maybe 12 hours later and wouldn’t get it back.
Modem is the XFinity X1 Gateway (Technicolor CGM4331COM or “XB7”) and has no trouble with the old version (1.5-rolling-202311210100) but will not get a reply for DHCP on 2025.3.30 build or any prior build i have tried. It has no trouble getting a lease when the modem isn’t in bridge mode, however, and is rock solid the entire time. Double-NAT is not the ideal setup, however, so this is not a solution, but merely a bandaid.