I think there are no standards for the things you mention because there shouldn't be any. If they existed, they would be necessarily complex and bloated like JWT or XML, introducing unneeded complexity in anything that adopted them. age tries to be as much complexity needed for the use case and no more. Other designs can make their different choices for their different use cases. That's good!
No contest on (1). On (2) I want to mention that "recipient" might have been an unfortunate choice of word, but it has nothing to do with email, it just means "a thing that can be encrypted to" like a public key, and any envelope encryption scheme will have something like that even if it doesn't give it this or any name. For (3) I have no idea what your requirements are, but I would claim age is pretty much state of the art (partially because it's not hard, age is trivial by cryptography research standards). Wireguard is being merrily deployed everywhere and uses ChaCha20-Poly1305 as well, FWIW.
(age does have a magic number in the header, and I see you came to realize why metadata in files is a bad idea. It's a bad idea also because it would be attacker-controlled.)
[Edit: s/No context/No contest/]