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At its oldest your phone is 13 months old, that's not just used, its unbelievably well used.
I did the maths, near enough every 1.2 days your used battery was charged and discharged.
If its a newer than 13 months, the charge rate is worse
my opinion... put stock android back into it and chop it in for a new phone.
If you don't know the history of the device you are gambling that components, screen, internal memory,etc, won't be as used and fail sooner that you expect. You might even help that failure along by opening the device and changing the battery.
I bought a used P6a with 22 battery cycles and have had it for over two years, and expect it to reach EOL, when I will dispose of it.
Had it been me looking to buying that device of yours, I would have passed.
I know its not what you want to hear, its just my honest opinion.