...he is an extremly patriotic Yeerk ... thought the empire was the only way the Yeerks could survive. If the chance to morph Yeerks was present he might have taken it. ... if it help their repute he spared his fellow Yeerks and put them over the plan.
Dude, can't understand what you mean here. A patriotic Yeerk, who would stand by the Council of Thirteen, then suddenly opting for 'permanently morphing' some other random species sounds a bit contradictory.
Unless all Yeerks are hypocrites.
A tyrant, as such are the ones that fall under the Visser ranks, would certainly oppose being what they are, which is YEERKS.
The keyword is 'tyrant'. Tyrants need followers. If the Yeerks under a 'Visser's subjugation suddenly became 'something else' who would 'said-Visser' have left to give orders to???