How to copy music content and playlists between iPods (Nano 5th Gen 16GB)? I'd like to mirror the music tracks/files and all the playlists from one iPod Nano 5th Gen 16GB
to another another (Nano 5th Gen 16GB). Is there a way to easily automate this process?

Details

My iTunes music library is much larger than the 16GB iPod, and I have hundreds of playlists. The selection of playlists/artists/songs to reduce down to fit on the 16GB iPod is rather tedious, even when playing automation tricks (like creating a smart playlist containing all the songs that I've played in the last year, or whatever).

I don't want to perform this tedious playlist-selection process every time I copy to another iPod.

Potential solutions (?) I'm investigating include the following. 3rd-party MacOS applications that claim to facilitate this (TunesGo, iFunbox). Maybe I can use iTunes somehow to copy from iPod to a temporary iTunes (MacOS laptop) Music folder/directory, and sync said folder to a blank iPod? Something else? I've tried rsync-ing (on MacOS) the entire filesystems from a source and a target iPod, with no success (caveat: I may not have executed the initial rsync correctly; I'm retrying that now).

Regardless, I'm here asking to see what processes have 1) worked well for other users (via empirical experience, not just speculative assertions), and 2) is easy -- ie, I want to minimize the number of steps in the procedure.

Authorization isn't an issue here - I own all the music and the iPods. I have several open "authorization slots" on my iTunes Store account (or whatever). And money isn't the issue here, so if I have to pay more to copy between the iPods, that's ok (within reason).

fyi, I really love the iPod Nano 5th Gen's... except that they're small. I don't want to go to a larger/newer iPod because I like the Nano-5th-Gen's so much.
Assuming I understand your question correctly, would a backup > restore process do what you want? 

More specifically, have you tried doing a full backup to iTunes of the first iPod and then restoring that to the other iPod? Basically, when you connect the second iPod to iTunes you can do a Restore from the full backup you just created from the first iPod. 

Once you've done the restore, the music may not be on your second iPod to start with, but then syncing it with iTunes should restore all the music/playlists you had previously downloaded to the first iPod.

NOTE: I'm also assuming that all the music, playlists, etc are stored in your iTunes library on your Mac.