I've made an app that makes it possible to schedule a post in Lemmy at an arbitrary time. It's available at https://schedule.lemmings.world and can be used by people from any instance.
Self-hosting is possible, though I don't have a direct support for that right now, you would have to figure it out yourself (it's not hard if you know how to work with the Symfony framework).
Awesome! I'm just starting my workday now, but I could take a look to see if I could put it in a docker container if you would like. I would have to do it after work, which means I probably won't make significant progress until the weekend.