Having a bit of firefighting experience with Bell 205's and 206's using the Bambi bucket....you are quite right about Max Power takeoffs.
My suggestion was at some point prior to the malfunction....lightening the load by mashing the bucket dump button would have gone a long way towards preventing what happened.
After the malfunction you are quite correct in your comments....that bucket full of water determined the vertical trajectory and then became that sea anchor you mentioned.
Also....if you find yourself right at the max limits on power each takeoff....you might want to rethink your bucket capacity and cinch that bucket in a bit to give you some "Uh Oh" Power Reserve.....before it becomes an "Aw ****!" deficit.
if the aircraft is too heavy....a simple button push puts some water back into the pond....where you can come back and dip it out on another cycle to the fire.
In the USA....firefighting with helicopters is less about putting out the fire and more into safely making money throwing water at the ground.
USFS Rule encourage that as they want to have as few crashes and loss of lives and injuries as possible during the Fire Season.