One thing I noticed- when you have the Saxon invasion trigger which gives Cantia a bunch of guys and causes them to have a war with Dumnonia, the AI tends to fail spectacularly at handling its new stacks, sending their old levies first against the Dumnonians. This result almost immediately in them getting wrecked and despite still having an additional 6,000+ troops, the 100% warscore auto-ends the war with nothing taken, which is a bit disappointing.
That'll probably be resolved by changing the way Cantia expands. Most likely an event will trigger to give Lancelot the Belgae, who will then usually swear fealty to Cerdic. This will trigger the Meonware Jutes moving in to settle the land. Once Lancelot is dead, the land will be ruled by a Saxon (a Jute) too. The rest of the expansion will be triggered by an invasion of event troops, which would carve out an independent West Seaxe from any land still owned by Britons in Dumnonia, preferring land closest to Cantia.