Crossing arms to skip songs, change volume, pause music is cumbersome while running.
Pausing music on traffic lights or whenever increased situational awareness is needed, is crucial and the current implementation on 645 (or any music smartwatch for that matter) is lacking. That said, having buttons on the watch makes it easier to control while running than the touchscreen on an Apple Watch or the Forerunner 630.
At the same time, dedicated buttons on an old iPod Shuffle or even the new Mighty:
https://www.engadget.com/2017/07/24/mighty-offline-spotify-player-review/
are more useful as you can reach for them without looking, but clearly those cannot be added on a watch like the 645 as each button needs to serve multiple functions.
I would thus suggest that Garmin adds support for BT ring controllers like this Sony:
https://www.designboom.com/technology/sony-waterproof-walkman-headphones-remote-09-28-2014/
I have been using it for 2 years now and to me it is by far the easiest and safest way to listen to music on the run. The ring is super light and you can control all aspects of your music instantaneously, which is the time it takes for you to react on a potential dangerous situation when running about.
Moreover, I would suggest that Garmin uses said ring to allow us to take laps and start-stop a run so that we don't have to cross arms when we do high-speed intervals on let's say the track.
I posted the "ring-controller" and many more "runner-specific" ideas on the comments around the new Coros watch on DCR's blog:
https://www.dcrainmaker.com/2018/02/first-coross-multisport.html#comment-2848523
https://www.dcrainmaker.com/2018/02/first-coross-multisport.html#comment-2848979
Hopefully Garmin will incorporate something like that to make music usable during a run.