I don't know if this helps but I have had exactly the same problem that I seem to have solved by doing the following. I firstly took off or disabled the "bloatware" that came with the phone. These were "wifi scanners" (your phone does that anyway), antivirus and anything else that was not relevant to me like "kobo book app" etc. I then used "stamina mode" when my phone was not being used, reduced screen brightness to 50‰,screen timeout to 15 seconds. I then drained my battery completely and fully recharged it. Since then I have had absolutely no drainage issues at all. If theres any software that's not relevant to you.... Lose it or disable it.
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