You should be able to direct the music app on the watch to look to the watch itself instead of Spotify. (Which is also weird because I didn't think Spotify was an available option, just iHeartRadio and Deezer...at least for USA). Anyway, this is done in the music app itself directly on the watch. If you dive into the music app and scroll around to the selection just below settings (which is source, but is currently just blank when I go there...also weird), then hold the up button to dive into it's menu, you can then select "Music Providers" which gives you several options. One is "My Music" and that will be what you downloaded onto the watch. You can also choose from the available streaming apps (on mine it is iHeartRadio and Deezer), as well as "Control Phone." I'm on Android...so can't help much with the Apple universe side of this.
Regarding the charging issue / battery life...there is a thread already running about this. Garmin has reached out asking for information such that they can troubleshoot and hopefully solve this. I am currently getting about 50% of the battery life that was advertised...but recognizing that my watch settings may be affecting this as well.