Check the list of autostart apps.
Select "restricted activity" in app options for background activity
Deactivate all location services, including the Google emergency/disaster notifications.
Use 60hz,lower resolution and dark mode
Check out accubattery
Did logcat change your insight to the cause?
First install or twrp format,etc.
Sometimes it is just that much more fast to repeat the install, than the diagnosis
Assistent is the worst thing to have on the Google suite.
There are so many functions and options for Assistent to deactivate. When completed the service seems like a hidden spy master.