#Amethyst is decentralised with no "company server" that can send posts on your behalf when you are offline.
There have been discussion around supporting scheduled posts at protocol layer, relay could be responsible for holding onto them, but I don't see an official NIP
This is true, but this could be solved by one or multiple scheduler DVMs. There is the shipyard DVM, but you kind have to trust the person running the DVM to actually post your note. Its a decentralized way of doing it, but technically it works today.
You can use the DVM on https://shipyard.pub/ already. As you use the same key, it will also show on Amethyst and other clients.
