The corresponding issue on Github has 381 thumbs up at the moment. It simply has not been implemented into the official Mastodon UI (as most of the others) yet. As you expected so far, this feature is only available in the backend, so you either need to use a tool that was explicitly created for scheduling toots (like FediPlan or Mastodon scheduler), or use a modern app that has implemented this feature, such as Soapbox on the web or Fedilab on Android.
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