Hmmm. Garmin states 8.5 hours with All GPS and music streaming (to bluetooth headset) for the 965:My Forerunner 965 gets 6 days with 1 hour GPS and streaming Spotify EVERY day. It has a big and bright always on display. No running to charge my watch for me. I know you wrote this post last year but my have times changed.
Forerunner 965 Watch Owner's Manual - Battery Life Information
Compared to 19 hours without streaming, they still seem to have an issue with streaming music costing incredible battery life. Either a terrible implementation of music replay and streaming or the chipset is not happy with this kind of stuff.
For me personally, the battery life parameter is not a big thing. Never cared if I have to recharge my Garmin after 3 or 5 days. Now, my AW7 is at the edge but the Ultra has enough for me. Running >100km a week, every day.
Have fun with your 965, it sure is an amazing watch. Saying this without cynism or so.