I've had this problem for so long, through different IOS versions. The only solution I've found is just what others have said - wipe the phone. I'm using an iPhone 7 and its struggling already, but the fact that my system takes 23gb out of 32 makes it worse. Every time I try to delete something the space is free for a few hours or a day then my system eats it right back up.
I'm getting really sick of this, my phone is literally unusable because my storage is so full (i.e. I can't send or receive texts, calling only works half the time, my apps are all invisible, they all crash, my wifi, data, and bluetooth barely work, boot loops happen all the time, all of my notes get deleted, and the list goes on and on). So basically if someone finds a solution other than the nuclear option I would be very appreciative.