by using adb bugreport and feeding the file to a locally running docker image / web service called battery-historian you can get pretty far in isolating probable causes. The visualization of the drain decline helps alot.
com.asksven.betterbatterystats is an on-device app that can help to some degree and show the stats in text form for users unable to use adb+docker.
I’m at Moto battery drain comparison helping a Moto G4 Play user and basically myself, as I have drain issues on a FP3.
Look for apps holding “partial wakelocks” and keeping the device from deep sleep / full doze. Check the difference turning off Wifi / mobile data makes in the changing rate of charge (steeper declines). It’s possible there is not only one cause.
(I’d love to somehow assemble “battery drain baselines” for devices for users to compare (between versions and devices). But it would need to happen with default apps. Individual settings and device external factors as bad radio signals change measurement reproducibility though)