I really like your detailed answer!
Your recommended level for tasks is great
but one thing to add, I go with a big team -as ED- and I can go to all the 3 bosses at level 130~ , the only thing that I'm afraid of is the monsters on the way to the bosses, so at a larger team mage with mana shield will have no problem at all. also, WZ 5 and 6 tasks are really hard on my level -130-