Hello,
We here at Agent Mobile just discovered something relating to this as we had a number of iPhone 6's in for repairs and deal a lot with these advanced issues. After replacing the battery we were stuck with this same problem (Battery replaced was OEM certified). We feel this is related to an OEM apple battery having the correct voltage and amps to continue the update. After temporarily putting the original depleted batter back in iPhone, the restore process went smoothly. So if anyone is having issues and you still have your OEM battery that is a place to start. If you have never done repairs to your iPhone there may be a deeper issue. Seeing this issue still discussed in 2018 we felt is was needed to mention our resolve solution.
Thanks,
Josh
Agent Mobile