And it seemed like we could have used a few more tanks due to the constant de-aggro mechanic.
For sure, probably a good idea.
We've only killed Magni once (haven't gone back for a second attempt just yet) and it took us a good 25+ minutes with a total of 7 tanks (3 protection warriors and 4 feral druids rotating in and out with aggro). I really don't see it possible with anything less than 6-7 tanks, the more the better (safer, rather).
Good luck!