5E CR guidelines are soft. I don't think extreme optimization needs to be assumed here at all, plus there's not much room for optimization from levels 6-11. There just isn't, and you don't even have access to the "big guns" of magic either.
A party of 5 fighters (who are neither complete morons nor total optimizers) could take this monster down in 5 rounds. They're going to dish out ~15 points per fighter, per turn, that's ~75 points per round, accounting for peaks thanks to Action Surge and crits and valleys from misses. At level 6 they will have a proficiency of +3, assuming they maxed their attack/damage stat that's +8 to hit minimum, so they only need to roll a 7 to hit.
The biggest danger here is the breath weapons, and the risk of melee-types simply not being in damage range. If your party is clever and the hydra is stupid, you can even get the heads to hit each other and 10d6 will, on average, nuke any other head.
So again, I'd give this monster about 5 rounds.