Have you used Live Tracking on any of those 6 to 9 hour rides? This is what the OP is wanting to do, track his ride. On my cross state rides, there are a lot of people that have Garmin computers that use Live Tracking turned on and not one of them has done a full days ride without having to recharge their phone along the way; usually by lunch time. My cross state ride is 2½ days consisting of a 65 mile ride and two 80 mile rides. Including lunch and break stops, the 80 mile days are about 8-9 hours long. Not sure what the OP's cross country ride consists of but I'm sure his days will be similar.

When I attempted a test ride using Live Tracking with my Bolt, I started with a fully charged phone, optimized for longer battery life, connected to the companion app needed for Live Tracking. It dropped to 10% battery life in about 2½ - 3 hours. Using the same phone for tracking and just running my cycling app, I get a full days use out of the app and still get location information back to my wife and family members.