I was having the same experience where I was getting super disappointing battery performance and was ready to replace it, when I read an article where the author claimed it is very unlikely the battery is at the end of its life (wish I could find it) and it is even more unlikely that a battery you buy off of eBay/Amazon will be better than the one you already have.

Turn off Google Fit, turn off always on OK Google hotword detection, clear the Google Play Services cache.