The problem is I can't find a solution and I don't know what what's causing the high activity
It can be alot of factors especially because you are using alot of xiaomi, google etc services in background.
If i would be on your place, i would firstly cleared dalvik and cache from twrp, after completely disable Google locations(in Google red icon on miui launcher), delete location history, disable Bluetooth and use for few hours and check how battery behaves. If nothing changed then disabled any unimportant xiaomi services.
Then disable unimportant Google sync services( Google fit etc) and checked if there is one with error symbol.
Next, disable google automatic backups and restore and xiaomi mi cloud.
Also xiaomi find device service.
This all thing disabling is just to see how much battery drain and standby activities are without it and additionally to "fix stucked services and features" after you can gradually enable ones needed.
When you did all of that, use for a half a day atleast to see hows phone behaves. If everything is fine, enable few features/services and check again if there isn't high battery drain.
Results will be, you'll have normal battery drain with all of needed services enabled or you'll find which is significant battery drainer.
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