What @Kerridoc said is probably your best option
I have another defense team to suggest, however, in this case you’d be forgoing Kashhrek’s awesome ability to tank (he’s probably the best of the 4* for that position) and instead, be depending on BT’s beefy stats and talent to cover you instead. Upside? Only running one healer, and making the best use out of BT’s talent. Downside? Not using Kash, cuz he’s tough to get through without Falcon or dispel
So another option to consider:
You could trade out Onatel for Poseidon for more speed, but Onatel + Proteus could mean your opponent might struggle to gain any mana as long as BT holds up. Plus, Onatel is pretty beefy too, so she’ll be a pain to knock down, especially with BT protecting her.
You could also trade out Sonya for Captain, since dispel is already covered by Caedmon on the left, but Captain only really shines when there’s minions, so not as dependable as Sonya would be.
Sonya and Caedmon are fast and dispel, so best in the left wings when possible (Caedmon even moreso with him being squishier, whereas Sonya can flank pretty well too).
Proteus is just too good, so he should be flank to ensure he fires; his mana control should always have a place on your defense.
BT center means everyone will get buffed before firing (as long as he holds his own).
As for who to level (just 5*)
Red: Queen of Hearts is definitely your best option; taunt can be super fun to play with, and can work really well when paired with other minion producers, too.
Yellow: This one is tough, because Onatel, Poseidon and Vivica are all great options. So this decision would come down to your priorities basically. Poseidon is probably the safest option, since he really needs to be at 80 to shine (since he’s mostly about sniping) and fast snipers always have a place. Onatel would be more for the mana control, but it’s not as straight-cut as Proteus’s, and since you already do have Proteus maxed too, not as crucial either. She’s one I’d only max after playing with her for a bit and deciding if her type of mana control works for you (cuz it does take a bit of thinking to master). She’s a pretty good defender, but does better, like any other mana controller, when someone is actually pulling her strings. As for Vivica, she’s an amazing healer, but slow mana can make her more difficult to depend on when compared to your other average speed healers. & Since she’s all about support, upping her to 80 would only be beneficial to you for defensive purposes, or to increase her survivability, since she isn’t an offensive hero.
So honestly, since this would be your first 5* going to 80, I’d probably do Poseidon since he is the most versatile, but that’s just me. If you value Onatel’s mana control more, love playing with her or value defense the most, then she may be the better option for you. As for Vivica, I’d hold off on maxing her til you have at least one offensive yellow 5* maxed, since her benefits from going to 80 aren’t as huge; her special works the same at 70 as it does at 80.
Hope this helps If you have anymore questions, feel free to ask
Edit: I should add, even though Kerridoc already covered this, would be to focus on more of your 4* first before spending the incredible amount of time and resources it takes to max 5*. They’re much faster and cheaper to level, and are typically stronger than 5* til 5* are about level 30ish (?) in 4T. So you’ll finish maxing two 4* in the time it’ll take you to max one 5*, therefore, you’ll deepen your roster even more.
I see a lot of great heroes in your roster, and would be happy to suggest who to level if you’re wondering, but since it seemed like you wanted to focus your questions on leveling your 5*, I wanted to at least get that out there. I’ll still throw a few suggestions your way (4*):
Red: Falcon (elemental defense down is amazing), Gormek (defense down is amazing), Scarlett (high attack stat ie high tile damage), then either 5* or Colen (great in some WRC)
Blue: I guess just finish Agwe since he’s close
Green: Finish LJ (great tile damage), then I spose Skittles could come in handy in some WRC
Yellow: Wu Kong (you’ll thank me when your titan hits fly through the roof), then maybe Li Xiu to deepen your bench and get some more mana control, then you can either go back to your 5* or finish off a few other yellow 4* instead
Purple: Might as well finish Sartana off at 70, then max Ameonna for tile damage, then you could work your way up to 80 with Sartana if you have the mats.