i always understood the poor mans copyright ie mailing to oneself PLUS remember all daw multitrack files in a original song are 'dated' by the OS was sufficient.
You understand wrong. Yea, they do it sometimes in movies and TV—even Perry Mason—but it doesn't mean a thing and never has. You have a self-interest in the outcome and nothing stops you from opening and re-sealing an envelope.
Disinterested 3rd party registration services
for Unpublished Works only are readily available and cheap. They don't look at your content. In the old days, they dealt with sealed envelopes only but this is the internet. Again, not required but it establishes date of creation and authorship.
In the US, the only compelling reason I know to use a
Form SR/PA Unpublished is if you are in the habit of revising your works or creating derivative works. The Published forms give you places to refer to earlier LOC Registrations. Still, you don't have to if those registrations were Unpublished (you must if they were Published).
I don't care what anyone does in this regard and whether I do it or not and whom I might use is nobody's business but mine and my partners.