I am having similar issues.. I have a iPhone 6S 128GB.. I was being prompted multiple times to update the iOS so when I finally had the space for it, I updated..
I tried updating OTA but had an issue and update didn’t push through.. So I deleted the iOS file downloaded on the phone and performed the iOS update using iTunes.. Update failed again and iTunes asked me to restore and update.. Then on the process still had an error restoring, so now my phone is stuck on recovery mode.. Tried this twice to no avail..
So I went to the Apple authorized repair center, and they told me the iOS must have a bug that caused the issue.. And they are charging me $40++ to restore my iPhone, and its not even guaranteed!! And they told me that if they can’t restore it, its most likely a hardware issue and they are gonna charge more and they advised it would probably better for me to buy a new iPhone!! Like ***??? This morning I have a perfectly working phone and now just because of an iOS Update, my phone is not working anymore!! And I need to pay God knows how much just to get it working again.. Really Apple, really???