Is there a way to schedule the creation of a topic? And to schedule a reply to someone else's topic?

[Lilly]

as per the linked topics:

@Discourse

This guide explains how to use topic timers to schedule a topic for future publication in Discourse.

Required user level: Trust level 4 users or Staff (Admin/Moderator)

@Discourse

Automations can be created and updated from Admin → Plugins → Automations, or directly from /admin/plugins/discourse-automation.