This does not really solve the issue. First, you would have to select everything you want in that playlist, which may not be everything you have on your library, this would be a tedious and boring job, duplicating probably years of curated playlists.
Secondly, you cannot, of course, add to the smart playlist by dragging and dropping or by clicking add to playlist, so you are limited to whatever choice you set which may not include a new item you may have added as it would not have been part of the original criteria (a new artists, a new album, etc)
Trying to just make a playlist offline seems to be ignored by my iPod Touch 6th gen, no amount of sync between itunes and the iPod moves the songs, if iCloud music is turned on on the device, it appears to ignore iTunes changes when it comes to music and wired connection.
Creating a new playlist and dragging the contents of one of my old playlists to it (in case there is something dodgy with the old playlist, I don't know, this has gotten more and more mysterious with time) iTunes notifies me that it cannot make the new playlist icloud enabled because it has files types that are not allowed, I read this as it contains apple music songs as well (it only has music files)
If I create a new playlist, not a smart playlist, the option to make available offline is not there.
In short, it is a mess of inconsistencies to get your music offline, I am still trying 2 weeks after moving to an iPod Touch so I could use my Apple Music subscription. Downloading from the device via wifi is slow, chaotic and broken, with inconsistent download numbers, automatic non-user triggered downloads, inconsistent display of offline icon on songs and constant 'unable to download song at this time' message, a mess...Google Music looks like an inviting alternative, got it on one of my old android phones, just to see if it was an alternative, after uploading my music library (which is 15,000 files but Google swallowed it up in just under 3 hours) I was able to replicate some playlists (it even took the playlist information from itunes) and download a 2600 songs playlist in an hour. I have trying to download 257 files on my ipod touch for a week now! and I have a fiber connection with 100mbp.
Sad, it used to just work