Playing small countries like Brittany and Wales is very difficult. Especially Brittany and Wales. Brittany was front and center to both Angevin, Norman, and French interests.
Trust the histroic stratigy of Breton and Welsh rulers by playing England off of France, and vice versa.
While allied to King William of England, it is good stratigy to submit to France, as this is the way to play the two off of each other. When France and England eventually war, you can choose where to send your armies... to the aid of England, your ally, or to France, your leige.
As Prince of Gwynedd or King of Wales, I have byzantine-cheated and forced a double allience before, and have been allied with the Kingdom of Scotland and France once. I do not think this a real cheat as it mimics the 'Auld Allience' formed between France and Scotland, and included Norway.
As Brittany, a simular stratigy for you would prehaps be to Byzantine cheat an allience with Scotland, or any kingdom level power, but you can only do this if your initial allience with England were broken by either a death on their side or yours. I think it horrid that you can not have more then one allience at a time, and think it should be fixed.
Downside: If as vassel of France, and you chose to rebel and not aide him and become independent, he will get claims on Brittany. I have to see what the economic value is of the Breton counties too, prehaps they are neglected. I do not know, for I know little of Breton economic values in the middle ages at the moment. Wales received an update.
As Brittany, it is a good idea to look for nearby territories to take, rather then a desperate Habsburg like creation spaning far off provinces that would be economically difficult to manage. Near Brittany, look for expansion into Galacia, Ireland (Munster prehaps). Prehaps even look to take the Aquitaine somehow.
If you become king level, then it is more difficult for other kings to claim your titles, though they still can claim your vassels. The claim on a vassel is determined by the economic richness of a province, and I do not remember what Breton levels are at honestly.
Hawise will be succeeded by her son Konan IV, who will rule over Rennes, Nantes, and Cornwall (Kerne). This is when you will get more income, and are able to invest more in your infastructure. Become king ASAP, and expand to nearby locations. I like to peck at Morraco on ocasion, Cetba and Tangiers for instance.