There are multiple people doing 8+ accounts. I think one person has touted the strength of mainly Warriors though I think Mages can make more sense for most. I think on the ISBoxer Discord one 8 boxer was talking about doing the harder part of some content with a buddy playing his main (so 9 total characters when stuff was really hard).

For your 8 accounts with only PMMMM so far it sounds like you are planning on doing something like the following for your 2nd team (which would make a lot of sense):

12345678
PMMMM
---PWMMM

I would definately include at least 1 Tank on your 2nd team and a Warrior likely makes the most sense (though a Druid or Paladin Tank could also work). The 2nd Priest can be good but you could also consider doing a different healer (Druid, Paladin, Shaman) with the advantage of the Druid and Paladin being able to use a single spec to either heal or tank.

Best of luck to you and very interested to see what you do with them at cap. I could see up to 6M2P or 7MP being helpful in some raid scenarios.