Yeah battery savers/app killers tend to be battery killers from what I've seen. They run nearly all of the time which in itself can drain a battery, then they close things that need to be open or cause big draws ramping up the CPU to restart things which creates both heat and a battery draw. Curious when was the last time you went a week without this app? If you're set on replacing the device it couldn't hurt just to see what the difference is with it disabled or uninstalled for a bit. I mean just for S&G, Science and Graffiti.
Keep in mind, you should take any performance reviews with a grain of salt, especially ones for battery life. Not only does everyone use their device differently making it a giant variable but the age and health of the battery is a variable in and of itself. So you're correct the age of the battery plays a part but to be honest from the way you describe your use it seems like you would be getting pretty good battery life. You use BT for sound which is a saver not driving phone speakers, you have WiFi off most of the time which is another consumer, and if you're in a strong signal area, where your device isn't searching for a signal constantly, you've wiped out 3 of 5 battery hitters. However, if you're not playing games or scrolling YouTube, Insta, or TicTok 24/7 I would think you'd be pretty good.
With you being as concerned about your battery life as you are, have you ever gone in and looked to see where your app killer comes in for battery use? On the S10 I think it's called, Battery and Device Care, in your settings.
Let us know what you find.