Feudal vassals stay your culture, while doges take the culture of the county.
I agree with you at feudal taxes, though actually you just gave me the idea and i love it.
BUT i disgree with culture, ......IF you marry the Doges with women of your culture or if necessary courtiers with the same culture and thus guarantee them keeping your culture until a culture flip might happen.
Though this way i tend to get all kind of good Doges but stewards.