I am currently playing in high Platinum / low Diamond range. I sometimes still wonder what could be optimized in regards of my Hero Roster and Defense Team. This is my Hero Roster:
I wouldn’t advise building your deck towards defence - build around attacking raids/wars, titans and events… It’ll net you more ascension materials and more enjoyment of the game.
Of what’s currently maxed.
There’s no glaringly obvious tank… And obviously you have two red and no blue 5* maxed so that’s going to count out rainbow for now.
Grazul is probably the closest you have to a tankable hero, but then you’ll wind up doubling your tank colour and that’s a bit of a no-no (plus IMHO you want Azlar at flank, not wing).
Just a thought, I have seen a heavily emblem Buddy work as a serviceable tank, but you lose Kingston. I have a team that has done okay at holding low plat consisting of Joon>Kingston>Grazul>Victor>Triton. I am weak at Max tank aswell. Sometimes sub emblemed Melandor for Kingston. How this applies to your line up maybe try raiding with a Kingston-Triton-Grazul-Joon-Proteus. Heavy hitters with light healing and mana/special control, you could always sub Rigard for Prot. Sounds formidable. Fairly new to Plat so this may be all misguided advice.