Hmm I did this right after upgrading to iOS 12 - may be why I never noticed any problems? It's worth trying anyway.Just thought I'd add my experience.
Have an iPhone 8 and a was similarly getting dreadful battery life - about 6% drain per hour whether I was using it or not.
Tried turning off new features such as Screentime as well as some other apps, but made no difference.
Looked at this forum and reminded me about draining until the phone switch itself off and then charging to 100%.
Did that and it seems to have cured it. Last charged to 100% at 11pm and still at 100% at 8.30am this morning.
Remembered that I had to do something similar for my iPad a while ago after a point update caused a similar battery issue.