Graphene OS is a very interesting option but it only runs in Google's Pixel phones for now, so I'm waiting for the day it could be installed in other devices.
You might be waiting a while. If you read the documentation I think they get into to the reasons for that pretty thoroughly. As a side note, I think the prices for 5's and 6's is pretty reasonable right now (they seem to have fallen a bit from a year or 2 ago).
