Honestly, I do think there will be a better ending than what I've proposed, but it's not going to be perfect, or at the very least, Asterion and MC are not going to get to live togheter forever. It's not really a suicide mission to gather ichor, it's just a matter of thinking of a way to transport it safely, Kota is a god, even if not as powerful, so maybe he could pick it up no problem. Also, much like the greek heroes turned demigods you've cited, taking down those monsters of legends isn't ever a feat of brute force alone, to stay in theme defeating them will involve cunning methods and solutions, it's not going to be something straightforward like making a bazooka, and the master's powers are very versatile in that regard. besides, the team does have plenty of mythical power, I don't imagine we will be able to defeat all 7 of them in whichever team configuration, so which ones you get to take down will probably be suitable to whoever is available.
Asterion is the stuff of legends and has water affinity given he is a creation of Poseidon. The though part here is the valley is going to immediately target and attack him, but at the same time, he can recover from any injuries that should have been lethal. And he is going to have the most understanding of this realm than anyone else.
Luke is nearly indestructible and can fly, with military experience from the world wars. He's knowledgeable with stars, so it could be a great help with navigation.
Kota is a river god, and dragons often come with exceptional powers, he is confirmed to be much stronger than he looks.
Khenbish has wrestling and cooking expertise, his bad luck curse might be a problem, but he could do good for a supporting role.
Themba isn't explored to much, and i'm not too familiar with grootslangs, apparently his phisiology can have some kinky perks.
Wolf has sharp senses and is a predator, the hard part is figuring how to make him help since he is an animal.
Robert can see souls and has extra high body temperature, even if not outright breathing fire, that can still be very powerful. As a lawyer, he also has in depth understanding of contracts which is a core mechanic to how the realm functions.
Nikos has the knowledge of how to navigate and survive the valley both from his time there and from his predecessors' teachings. He might know some tricks to the realm even Asterion doesn't.
Pedro has his feathers with can be really good for monitoring and investigating from a distance, he is also adept at rites, even if they are not the flashy kind of magic.
Oscar also has water affinity, and brawn, perhaps naive but at least the valley might not actively try to kill him like it would Asterion.
The cobalts are also there, and being small would make them both sneaky and capable of reaching hard to get places. they might be infatile but should be able to do excellent teamwork when it really matters.
There's also going to be 3 others who should be able to join the hotel. so there still is more potential to join the table. I think one of them is going to be a medic too, and healers are invaluable to any party.