I'm having a similar issue, but I have a few things to add that might improve your situation.
There is a separate docker image for plex pass:
plexinc/pms-docker:plexpass
Also I have added a device to the configuration, which allows me to pick my GPU:
Again I'm having the same issue, except for me sometimes I'll open up a movie and it will work my CPU pretty hard without hw encoding, so I'll close it and open it again, and it will run just fine hw encoding (at least what I observe, it isn't hitting the CPU hard).