Your posts, captions, dates and pillars live in your own Notion. Cancelling doesn't touch them, and neither does closing your account. The photos and videos you upload are stored with us, and each post in your Notion links to them, so keep your own copies of anything you'd want to take with you.
In Settings, under Your data, you can download the records we keep about your account (your Notion content isn't in it, because it's already yours), or close your account. A closed account is kept for 14 days in case you change your mind, then deleted. Closing doesn't cancel your subscription, so cancel that first.