It's interesting how everyone seems to use their player in a different way.
I'm still having trouble grasping what exactly you want to achieve.
I don't even understand why you'd want every single track in one playlist. Surely the whole point of playlists is to play a specific limited selection of your tracks, based on some particular concept, theme, mood, etc? I don't think RB could cope with a playlist large enough to include all 120,000 tracks on my player anyway!
Me, I have hundreds of playlists in my media manager, including the 'all music' one, and then sync them along with the tracks to the player, when adding anything new.
Rockbox is, as I understand it, based on dynamic playlists. You create them in various ways, including by simply selecting a previously PC-created playlist, which RB will create a dynamic list from - shuffled if shuffle is turned on. Or you pick a folder or album or something and rockbox makes a dynamic playlist out of that.
I guess if you want to 'play all', you could maybe just have everything within a top-level folder 'music', and just select that folder to play. Maybe it would need some other setting set, 'play folders recursively' or something - but I don't know exactly because I never play tracks that way, someone else would have to explain that more precisely.
Or you could, assuming you build/update the database, look at the all tracks listing in the database and select one and start playing it. I think RB would then play all the other tracks.
Edit - here you are, I've read the manual for you! (reading the manual is, as everyone knows, only something to do as a last resort).
You can control whether or not Rockbox includes the contents of subdirectories when adding an entire directory to a playlist. Set the Settings → General Settings → Playlist → Recursively Insert Directories setting to Yes if you would like Rockbox to include tracks in subdirectories as well as tracks in the currently-selected directory.
So, do that, then select the top-level music folder to play all its contents.