When you use set an Alarm in clock app, you get to choose mutliple tones and even songs from Apple music.
However, when you set a Sleep|Wake up alarm using a sleep schedule, you are prompted to choose only limited tones.
This limits the Alarm tone options for sleep one. Hope, Apple lets users pick the full library of tones in next update.
This isn't the place for suggestions.
Please raise a feedback report at https://www.apple.com/feedback/
