I've made an open source tool for scheduling posts to Lemmy, you can find it at https://schedule.lemmings.world. It can be used by users from any instance and it can be self-hosted if you wish so!
Changes since the last time I posted about it:
you can now login with 2FA enabled
you can schedule pin and unpin of posts in a community if you're the mod
you can schedule pin and unpin of posts in an instance if you're the admin
when creating a post, you can choose to pin it to the community (if you're a mod)