I am on a very similar journey - good luck! WhatsApp is going to be the most difficult to shift I suspect, but I’ve changed my pic to let people know to contact me on Signal instead so let’s see if that helps!

In practice there is no way non-US intelligence agencies are sharing intelligence that is critical to their agents’ safety or national security with the US. I doubt they ever have shared that intelligence with anyone, save in very discrete circumstances. In public they will continue to pretend that they’re sharing, as they have always done.