doesn't that stop iCloud syncing, at least on my end? I understand I can't control what happens on the other end of the conversation but that is all I need to do on my end, right?
If you choose to have some of your data in iCloud, it is transported and stored encrypted. However, one of the keys is escrowed in a HSM cluster for an audited recovery process.
This is how you go request access be restored via technical support with Apple. This is also how surviving family members can get access to photos and the like (requiring a court order, at least in the US). Since they have the key, government entities can request access within the extent of their respective local laws.
If you turn on the Advanced Data Protection feature, Apple no longer has that key escrowed, cannot help with account recovery, and can no longer give out a key they don't have.