Sure, but can you find an NSA-designed backdoor in the source code?
> you can get it once and disable autoupdates
Try doing that with Signal, and you'll be unable to connect to the main network in just a few days because you get out of sync. Also, what do you do if there's a high severity CVE on the program? You still don't update or you re-audit all the new code?
What you describe may be possible for an intelligence agency, but completely out of reach for an individual.
> unless Apple is colluding with them
Given the most likely adversary is the US intelligence with a warrant, it's absolutely not far fetched to assume that in your threat model.
> you can at least be reasonably confident that it's not specifically backdooring only you
That's not really reassuring…