I currently have the following damage dealers fully leveled. Athena, Marjana, Drake, Sartana, and Liana
My support 5s are currently Khiona, Evelyn, and Vivica
I have enough mats to level another blue with Magni or Isarna
I have enough mats to level another red with Mitsuko, Zimkitha, or Azlar?
Would any of these help my team or should I save the mats for the future?
Seeing you already have Athena, I think Isarnia would be overkill. If you don’t have a maxed Alasie then Magni would fill the blue sniper role nicely. I have both Isarnia and Magni maxed and they are my blue stacked harbingers of doom in raids. I would believe that Athena and Magni would serve the same purpose.
As for red, I would lean toward Zimkitha for the fast purification and attack up. Azlar is a beast, literally and figuratively, in events and when you can use mana potions. I don’t have Mitsuko but that special skill reflect is a p.i.t.a. not to mention the mana down which is awesome for offense.
Saving the mats is definitely an option seeing that you have a pretty stacked 5* team. Also saving mats helps me focus on 4* s for a while and max those. With the different trials nowadays there are definitely benefits to having a large 4* stable.
Not sure if any of that helped 
For your blue decision:
You have quite a lot of snipers maxed, but also have Athena, so choosing between Magni and Isarnia is a little more difficult since neither bring anything new to the table. You already get your defense buffed by Vivica, have Marjana, Sartana and Lianna to snipe for you, and have Athena to lower defense.
Honestly, if I had to choose one it would be Magni, since fast snipers are always welcome, but I’d be more inclined to wait depending on how many pulls you do.
For your red decision:
This really depends on your priorities.
Zimkitha is nice for her versatility; her cleanse and attack buff are the best things about her, but that’s only really useful for raids. Otherwise, you can use dotes and bear banners, BT or Kiril to replace everything that she does. So if you’re big into raids, and want an overall solid utility hero, you could go Zim. Although, most of what she does works just as well at 70 as it does at 80. Only thing is her attack won’t be up to par, but that definitely isn’t her highlight. She’s got enough health to withstand a few hits at 70, but if you want her on defense, she’ll need to be at 80.
Azlar is a beast. His attack stat is crazy, and DOT is insane. He’s the type of hero that needs to hit 80 to reach his full potential, so if you’re looking for the hero that will get the most from maxing, it would be him. Nice tile damage for titans, but a little slow to use in raids. If you can handle the slow though, his damage does pay-off.
Mitsuko is just fun. If you don’t care about versatility, and are more about having fun with the heroes you play with, I’d go Mitsuko. It’s amazing watching blue heroes kill themselves in raids, especially ones like Magni, Athena and Isarnia. Although, her special doesn’t get much use outside of raids. Definitely a niche hero, designed for offensive raiding. Works well on defense too, due to her tankiness, but the reflect part of her SS might not be as effective.
Personally, I’d go with Mitsuko, but that’s because I like unique heroes; they’re more fun to play with. The more practical options, if you’re looking to grow, be competitive, and have more versatility, would be Zim or Azlar.
In short: Azlar for offensive purposes, Zim for her utility, or Mitsuko for fun mostly
Magni and Zim for me.