Back to my keyboard now so I can give some more detailed advice.
And this is why I always recommend leveling a couple of rainbow teams of three-stars first. Especially with the completion rewards for challenge events potentially jumping even more, you want a team that can complete all of them, including switching out the reflected color for another hero in the color that is strong against the reflected color. The good news for you is, it looks likely that the first re-vamped challenge event will be Avalon (reflect blue; weak against green). Doubling up Mnesseus and Hisan would be a great tactic to finish the rare tier of that event.
More good news is that you have some excellent three-stars, and many of them already maxed with emblems. You are set for your first two rainbow teams with Bane and Melia in yellow (great choices!) and Hisan and Mnesseus in green (also great choices!). In purple, Balthazar was a good choice as well. I’d go with Tyrum next, as he is fast mana and a dispeller. A compelling case can be made for Gill-Ra, as she’s the only three-star cleanser, and her debuffs are great, but she’s slow mana, and does no damage. Don’t ignore Chochin somewhere down the road, either.
Your real problem areas are red and blue. That’s not surprising, as I’ll bet you drew Boldtusk and Kiril and stopped leveling three-stars in those colors. Since Boldtusk is stuck for a while, it shouldn’t be hard to convince you to level some red three-stars now. You’re closest to done with Azar, so I guess I’d finish her, even though I don’t love her as a hero. Namahage would ordinarily get my vote for being your next project, but since you don’t have Belith, Hawkmoon needs to be your priority here. You need an all-team healer. In blue, Gato is great; I’d make him a priority. Valen is, imo, the best blue three-star, but if you don’t have him by the time you finish Gato, Gunnar is also an excellent choice.
Back to green and yellow. You don’t really have a lot of other options in green, and Brienne is far superior to either Friar Tuck or Isshtak. Also, be watching for Belith and Berden, who are both worth leveling. Only level Isshtak and Friar Tuck if you need them for class trials or war depth. In yellow, Kailani is a poor man’s Gunnar, but she’s a cleric, if you need that for class trials. Otherwise, with Bane and Melia (the two best three-star yellows, btw) already comfortably leveled, I’d take Jackal to 3^60, at least. His defense down against yellow is a unique skill, and he will be useful for you for a very long time. Chao is one of only a handful of easily obtainable rangers, so he’s also worth leveling, but not before Jackal. I’m jealous of both of these, as I haven’t gotten either yet. BTW, seeing Needler with 10 emblems makes me super happy. Needler was my homeboy for a long time at the beginning of the game!
Finally, don’t worry a ton about your defense team right now. All defense teams lose a lot at basically any level. What you need is depth for war and color stacking, and the best and fastest way to build that right now is with three-stars. I do agree with those who said to use Boldtusk as your tank, though. He’s quite sturdy. You might consider going with Gunnar at tank at some point, though. For a while, at your level, he’s quite good there, and Boldtusk also makes a good flank to him.