There are a few standards. OMEMO for group chats, though that, of course, requires support in the protocol itself, unlike OTR or PGP.

The Signal protocol is the de-facto standard for E2EE, and it works just fine even in large group chats. But you refuse to accept this reality. The Signal protocol is used by so many apps, obviously Signal itself, WhatsApp, Facebook Messenger, Instagram direct messages, Google Allo (back when it existed), Google Messages (RCS), Skype, Wire and many others. MTProto is developed by Telegram, only used by telegram, not properly audited and full of flaws. No one should actually use it. And the fact that it doesn’t support group chats is a design choice, because ultimately Telegram doesn’t give a fuck about their users privacy or security. They have repeatedly worked with governments and worked against the interests of their users. Their funding is also pretty unclear and shady, and the entire company just appears scummy. Give me one single reason why anyone should use this trash over a proper E2EE messenger like Signal, Threema, SimpleX or Wire.

Uh you appear not to understand how encryption works? Either something is end-to-end encrypted, and the service provider doesn’t have access to the encryption keys, and thus can’t read the messages, or it is encrypted in transit, the keys are held by the provider and the messages are decrypted on the server. The latter is exactly what Telegram does, even though they falsely try to market it as something else.