I got the following thought from reading your request. Not sure if it’ll help you enough but maybe it’ll give you some hints…
I use the goingrunning plugin by @jakabadambalazs to create different playlists (for various occasions) and I’m almost certain that you can use it also to create one (everything in your Beets library) playlist.
On my server I have a Madsonic 7 beta server running (Subsonic will also work just fine)
I place the playlists that the Going Running plugin outputs on my server in the Madsonic playlist folder.
And have set Madsonic to automatically import those playlist on changes.
On my iPhone I use play:Sub (but any Subsonic/Madsonic app will probably do) to connect to the server and download specific playlists offline. This last step is a manual action so not fully automatic.