I followed this post over the weekend as a P9PXL owner and I too have been experiencing high battery drain.
Outside of the many valid points on this thread and the post by @MetropleX above that outlines some possible work on the issue, I wanted to share some app related items I've found on my device.
First, the Signal app. In my battery usage setting page, Signal was the number one battery drain, at 40% with over 9 hours of background usage and a distant second was Home Assistant at 20%. I was able to reduce Signals background usage by about 45% by removing conversations in the apps setting page. On that page, one of my individual chats and a group chat were listed as conversations - I removed them, no other changes to Signal. BTW, this does not remove the conversations from Signal, it removes them from quick access or from being pinned to the home screen. Interestingly, I never pinned any conversations to the home screen, so I am not sure how these two were added to the conversations in settings. Possibly an errant tap of mine.
On my latest full charge, my Signal battery usage is down about 45% as compared to my previous full charge stats. Currently Home Assistant is sitting at 20% usage and Signal is at 18% after approx 36 hours off charger, overall battery is at 44% remaining. FWIW, all other apps are well below, email is at 8%, VPN at 7%, launcher 4% and browser at 4%. Web browsing is, by far, my #1 active screen time app since I have a large number of PWAs.
For Home Assistant, I am sure sensor updates are keeping that running in the background. But that was not the big drain. Are there other possible culprits? Maybe, probably - as previously stated, there are too many variables to narrow this down to a single thing. But Signal has consistently been 40-50% of my battery usage, being able to cut that in half is a huge win. I hope this helps others.