Not doubting this whatsoever, but what are you citing here?
Per the Snowden slides, this (FAA702 collection) is the #1 most used collection method by US spies.
They can basically read approximately every camera roll and iMessage in the country with a few clicks.
All of the iMessage access data (either the iMessages themselves or the iMessage sync key for “Messages in iCloud”) is stored in the non-e2ee iCloud Backup.
This means that the iMessage service e2ee is meaningless because the iMessages themselves are not e2ee.
Note well that turning on iCloud e2ee is insufficient, as everyone else that you iMessage with will still have it off, and all of your iMessages will still be backed up non-e2ee to Apple in their device backups.
Having iCloud e2ee available is the best of both worlds for Apple: they can say that it’s available for privacy-conscious users to opt into, while still being able to turn over basically 100% of all iMessages to the feds whenever they ask.
I do understand the counterpart party problem, it is not someone can control. Sent once, it's free to be seen by anyone.
With an actual warrant or wiretap order, there is no guarantee that Apple can't or won't insert additional endpoint keys into the conversation.
But, that said, 99.9%+ of users don't have e2ee enabled for iCloud Backup, so the e2ee in iMessage is mostly irrelevant, and it's available in full to the feds at any time, no warrant required.
tl;dr: use Signal.