Why the Buccaneer was a poor fit for COD and tanker duties

Would proposed Buccaneer variants have suited carrier onboard delivery or tanker roles?

Not especially; the Buccaneer was optimized first for low-level anti-ship strike and later for nuclear strike rather than carrier logistics or tanking.

The Buccaneer was a poor basis for carrier onboard delivery or tanker service. The original aircraft was shaped for low-level attacks on Soviet Sverdlov-class cruisers. Its later Mark 2 form was enlarged and improved for the nuclear-strike mission. Those priorities produced a valuable strike aircraft that was poorly optimized for carrying stores or fuel in a support role.