It’s interesting to hear West Marches style mentioned as a campaign length. I always thought the campaign here just meant a collection of PCs in a place doing adventure of the week like an episodic sessions until everyone decided to stop. There is no real designated starting or finishing point.
For me campaign has come to mean a setup of intention. The meta plot is known by all and interacted with in every session like a modern serial program. That connection point to the meta plot is what makes it a campaign in the way a setting sandbox makes west marches a campaign.
What’s new to me here is taking episodic west marches a putting a session timeframe to it. Nothing wrong with that, just something I’ve not really considered. Since I’ve moved away from the style it’s not surprising though.