I's suggest making the interrupt mechanics more dynamic.

At least allow casters to gain a buff each time they are interrupted in a manner that is from dmg. So if they are dealt 15% dmg then the next time the melee would have to deal 20%, then 25% then 30% up to 40% but once the caster does get a successful cast off the buff would wear off. But also make this buff last maybe 10 seconds, so it can't be something that they run around with for a few minutes and use to cast a nearly uninterrupted sleep or such.

I just do think that making it a stagnant 15% may make casters even more of a target because now players would have a known factor to saying, ok if both DD roles focus the caster they will never get a chance to cast. Cause say at 9k HP you only need to deal about 1350 to interrupt them. So if two DD just pummel the poor caster, the caster will never get a chance to cast but if you at least increase the interrupt treshold each time they fail then you at least give them a fighting chance to get out of being ganged up on like that.