Best of luck with this! Been lookin for a horde group.
So far I think the consensus is that mythic Nya is too scaled now after squish, so even being 60 doesn't give much of an advantage. Highest I've seen a group go before fallin apart is 3rd boss, both geared 50's and 60's. The Phoenix Community that usually sells mythic raid/dungeon carries is not even offering a run for sale until next raid release when iLvls bump up