Dear goodness, playing as Bretonnia (Bordelaux infact) in the campaign is just painful. I've managed to confederate all provinces apart from Bretonnia itself (I'm leaving them as my ally to defend against the Norse folk). Anyhow, I find I never have enough money to raise a good enough army as the cavalry is just so expensive to purchase. My mixed forces get absolutely butchered as everyone runs away at even a squeak. Very frustrating :/
Is not that difficult
I managed to finish my campaign with Bretonnia having tried numerous things first.
In my opinion go for agricultural economy. At first is slow, but after you research the relevant techs very powerful. And build the two buildings needed everywhere. You do not need walls on your cities. Third building on the secondary towns should be the taverns giving you control boost.
It will allow you to run 4 stacks of 12/13 of various Knights, 4 Archers, 1/0 Paladin, 2 blessed trebs with ~12 towns (Bretonnia has 14 towns in total) plus trade with Empire and one more.
Plus maximum amount of Damsels.
Also send a Damsel asap to find the Dwarves and make trade agreements with them asap. They are reliable income sources. As is the Empire.