After updating my wife's Xs Max, she started having extremely poor battery life. Off the charger at 7-7:30 and almost dead by 10:30-11, the battery health was either 92 or 93%. Honestly I can't say what it could have been, I really didn't t/s it, upgraded her to a 12 PM, made her happy.
I wouldn't have done the 8+ either, you're smart. That's a little steep for that phone, nothing wrong with it, just not at that price. I think you'll like the 12, it's a solid iPhone. Just takes a little time to adjust to not having a home button.