Most users' "threat model" is loss from actually losing things, or doing dumb things to themselves. They expect Apple to fix that.
Apple understands this, and in most markets there's a Genius Bar somewhere near the user, with technology letting Apple help them.
If your model is something else, they also have your back.
> remember that enabling advanced data protection just means they'll get your conversations from the other partys' iCloud backups
Conversations may have a counterparty not using ADP, your data storage probably doesn't.
And yes, who else can see things is very important. People show others "your" messages on their phones all the time, the more unfortunate the message, the more likely they are to overshare. Very much worth remembering they have copies of the same discussion, for this, and for backups.
While ADP won't solve betrayal of trust through analog sharing or digital resharing, Apple DO have a way to ensure your message is only between you and a personally verified counterparty:
- iMessage Contact Key Verification: https://support.apple.com/en-us/118246
After that, trust is up to you, or use a different app – knowing it can still be shown.