Mastodon has a built-in official post scheduling system but (for some unexplained reason) you currently need to use a third party interface to access it.

The third party Android app Fedilab includes access to the official scheduling system, and its makers @apps also have a site called FediPlan at https://plan.fedilab.app which should work on any platform.

If you would like an official interface for scheduling, and if you're comfortable using github, give a thumbs up at https://github.com/mastodon/mastodon/issues/10088