In case this is helpful to anyone here regarding mobile options currently (although these options may be in some jeopardy now that Apple has just announced ‘the end of iTunes as we know it’, starting with the next Mac OS update; it will depend on whether the Apple Music options that replace it still allow the same functionality and whether that will still work with Roon; it sounds like they probably will still work similarly):
I rely completely on Roon at home and my mobile solutions have been very satisfactory. My approach has been to keep my Roon library in iTunes at home (in my case on an SSD in my Nucleus) and just have Roon watch the iTunes folder. I do all my organizing and playlist changes in iTunes. I add a few extra tags and things in Roon as a bonus. Basing my Roon library in iTunes this way allows the following two easy mobile options for shifting from my Roon setup while on the go. One of these options has quite good sound quality for a small investment, and one has superb sound quality for a larger investment:
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For very good and cheap mobile playback: I subscribe to Apple Music for $10/month. Apple syncs my entire iTunes library and playlists from the Nucleus SSD to Apple’s servers, including my albums that are not available in iTunes (for me, Tidal doesn’t have enough of my music, and it’s hard to sync playlists; haven’t tried Qobuz yet). All my albums and playlists are then easily available as decent-enough AAC files for streaming or download on my iPhone. In my backpack I keep a DragonFly Red, an Apple Lightning to USB 3 Camera Adapter (be careful not buy the older one without the extra charging port, it doesn’t work well with the DragonFly), and a nice pair of $200-$300 earbuds that block outside noise.
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For superb but pricier mobile playback: I use a Chord Mojo+Poly with a 400GB microSD card and pair of high-end audiophile earbuds that also block outside noise. It is a relatively simple matter to take the same iTunes playlists I’ve created (that are also the ones that appear in Roon), and from the iTunes window on my laptop drag ALAC or AIFF files, including hi-res files, into playlists in Doug’s Apps M3Unify running on my laptop and thus export them from the Nucleus SSD over my local network to the microSD card, which is plugged into an adapter on my laptop. For playback control of the microSD card in the Poly while on the go, I use the MPD player mode in the very good Glider app for iPhone (which has a few quirks; for one, be sure no song titles begin with letters that have diacritical marks). This is a terrific solution that in some ways I even enjoy more than listening through Roon on my main rigs at home!
Of course, some people have other needs or other setups, but after some initial trial and error these options have worked really well for me for now.