I suppose there’s already a number of suggestions addressing those various features…
- download playlist for backup → duplicate
Export music playlist to m3u+media - download entire library/all playlists to plexamp → duplicate
Download entire library or playlist in Plexamp - share individual playlists w/ other users → implemented
https://support.plex.tv/articles/shared-media/
Not aware of an issue where playing a playlist stops after 4-10 songs. That sounds more like an issue/bug…