Messenger seems to be properly multi-device, but you pay for this by some PIN code bullshit (maybe they removed that, I haven't seen a popup about this for over a year now?) and having to sync chat history in the background, through a process that is, of course, broken and unreliable.

I'm actually still jaded about this. Messenger worked fine before they broke it by introducing E2EE; it took years for them to fix the problems this caused (at least the ones that were immediately user-perceptible).

I don't know, I don't use those. It is for Signal, I don't think so for Instagram, since I don't think that encrypts end to end.