Wow, super lucky 2x! Candraphon is a big upgrade over Athel as nuker, Minaya is a strong healer with speed aura, some of those epics are decent as well though maybe not anything you immediately needed. The pair of them are your next 6*s I would say, though not sure on using the legendary books - definitely not on Candra, on Minaya of course you'd like the increased heal and reduced cooldown on her A3, but it may take a lot of wasted books to get there. 

For revives, if I see it right Cardinal is already at 6*, so why not just continue with her? She's obviously miles better than Shaman and not necessarily worse than Jorrg or Melga - don't underestimate the advantage of her reviving people with full TM. You may not actually need revivers everywhere, though, if you bring Minaya.

I see what you mean about lacking CC/AOE damage on dungeon waves, you have a strong roster but not too much in that direction indeed. Ideally you'd want two people putting out AOE damage, not just Athel (or Candra moving forward). A team with Doompriest, Apo, a reviver plus Serris' AOE decrease attack, then only Athel/Candra as DPS is just overkill on support.  That's the downside of using Serris in dungeons - a Tayrel or Stag Knight would also put the debuffs but do serious damage in the process. Since you don't have them, you could look at using Athel and Candra together, other than in FK/Spider where Coldheart is already the second DPS, though competition gets fierce for the remaining slots then. 

For Spider, you could probably still progress a few more stages by simply nuking the spiderlings plus firing Heartseeker at the big spider. Candra will help with that. But before too long, you're going to have to change your approach and get additional turn meter reduction on the big spider. An Armiger could help with that, indeed, or that Soulbond Bowyer (AOE A1 plus turn meter depletion, like Coldheart's, on her A3). 

In regards to revivers, Cardinal is 6*, yes, mostly because I had no one else to rank up at the time. The reason I'm leaning towards Jorrg over her is that he has a more offensive kit, with turn meter filling and attack buff and a chance of HP burn on bosses, while Cardinal has a defensive one with her heal and defense buff. She also has poison instead of a HP burn, but a much lower chance of applying it. I feel like I'd benefit more from having an offensive reviver when I already have Apothecary to take care of the healing. The problem I see with Minaya is that especially more glasscannon-y champions like Athel and Coldheart are vulnerable to burst damage in the dungeon waves, and all the healing in the world won't help if you're dead. Scyl is still 90 days away, so waiting for her heal/revive/AoE kit which would be a good fit for me is unfortunately not viable right now.

Cardinal's best place in the game is the arena, together with a specialized team that takes advantage of her revive with 100% turnmeter. You can use Cardinal instead of a speed hero like High Khatun together with a typical nuker combo. Cardinal needs a lot of HP/Def to survive the first hit and an Immortal set as protection against stun/freeze/daze. The opponent goes first, Cardinal's teammembers die, she revives them with 100% turnmeter. 

The advantage over a typical speednuker team is, that the attacking champs don't need any speed (exept one or two substats to manage their own turnorder after revive) and can use full damage gear, including boots. 

I've given the Cardinal arena team a try, the problem is mostly that either she dies due to lack of stats, or my team comes back and then don't have the damage output to take out the enemy team. I think I need to build a more specialized arena team for that to work, right now I just don't have the champions to make a dedicated team for just one area of the game.

Honestly, Cardinal + Candraphon is probably enough. Just fill the remaining spots with a champ that has AOE DEF down and AOE ATK up  

Scyl is definitely a great dungeon carry. In the meantime, if you take the Minaya route, you'd indeed have to make sure nobody dies, but since you'd have her speed aura as well as Serris putting decrease attack on everyone, that should help a lot in avoiding one-shotting on the waves. But anyway, Jorrg is not a bad pick either, a sort of epic version of Arbiter. 

Serris AI has a tendency to waste her spells unfortunately, so her attack debuff isn't always as reliable as I would've liked. It'd probably work if I did everything manually, but I don't have enough time to waste on managing every battle.

I might give the Cardinal arena team another go. I'm thinking her, Candraphon, Athel and Serris. Candraphon has his own damage boost, as does Athel to a lesser extent, and Serris can both remove any defense buffs and shields, and apply her debuffs. Hopefully that's enough against the squishier speed teams.

Candraphon's damage boost is more of a cleanup spell. You don't want him using his A3 first, you want him using his A2 first, and you want him to hit as hard as possible when using it.

Serris is a good bet, for buff stripping and DEF down. Athel is good for now, but you really want to be replacing her with someone more reliable. None of your current champs fit the bill, but as you get more, she'll be the first one on your team to replace.