Or how about if damage slowed the convoy?

Then the convoy would start off going flat out like a convoy would if it's going through a dangerous area. Attackers would need something quick to start hurting it.

Once some damage is inflicted the whole thing has to slow. More damage makes it slower. Then the other tactics start to come into play. If a defender is able to execute repairs it would speed up again.

make convoy engines an actual seperate target