Trying ot keep the other bretonnian factions alive is the worst possible idea as they are the weakest factions of the entire game with a lot of much more powerful enemies around or even in the midst of them.

Try to blitzkrieg Kemmler. You need to be fast, cause Red Duke(?)dont know how hes called in English, will fight you whith a strong army. I build garrison building in the minor city and raise a second army at least to buy some time.

Alternative you wait for for the Duke whith a garrison and Louen( in ambush) to beat him.
I had so much fun whith Louen, your first couple of rounds are one of the hardest. But after Kemmler, Duke are gone you can fight the deamon or whatever you want.

Eco was a waste of time for me, defense military replenishment were my prio.
I rebuild my cities after that, usually I dont play like this, but whith the French Brits it worked for me.