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.
I've always loved the irony of needing a Google phone so that you can run an OS the de-Googles your phone.
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).
IIRC the devs are only supporting Pixel devices because they are at least vaguely sane.
Supporting random other devices would be a nightmare.