The behavior between clients differs here. I don’t know of any app that will autosave a draft for you, but some keep the text in the text field even if you leave the app or it crashes. I just tried in Gluon and Micro.blog for macOS and they will happily let you continue writing where you left off if you quit the app and open it again.
But I agree with you, caching the next post in the web client until it’s published would be a nice feature.