I also like to consider my self an Abyss main, with mag as a close second, have taken both to S several times. And ya said you were struggling so I just started with the basics.
Anyways the worst thing about Cass is the fact that she is either really aggressive, or super control. So at turn 0 we sort of have this weird coin flip, do I mull for an agressive hand and go face right away…or do I expect aggression and go the usual slow game.
If we make the wrong decision we get punished. I usualy expect it to be control since that is the worse match up and choose aggression. The deck is good at adapting and switching modes but that is still a hardship that needs to be over come.
Sunsteel is more of a midrange pick, but with Kron out he may start showing up more as a valid alternative to taygete, or harvest/meta. But anyways, don’t retreat vs a Sunsteel you want it dead early as it is a minion that will snowball and pull value from not being a prioity removal. Remeber while they take big chunks out of you with rush, they are still ultimately slow so you can afford some health loss early.
…now here is a weird peice of advice that I am reluctant to give, because I really hate that turn 0 coin flip…you can totaly capitalize on that mind game, as a control deck you can pretend to be aggressive early on which will actually force a good keeper magmar player to pull back defensively. It’s a very strange match up that mostly comes down to out positioning, good replacing, and planning ahead, and a huge part of it is a weird mind game. Now your often in a similar position because maybe they are not keeper magmar, but your very good at pulling back if they stay aggressive.
Alternatively you could actually be the aggressive Cass type deck which, while in my opinion is not quite as strong as control, is just a consistent race since the keeper magmar will usualy start aggressively, putting it at a slight disadvantage vs aggro Cass.
Next I would try to give some constructive criticism on your lists but A. You have a lot of them so I am not sure which one is current, and B. “Experienced players” rarely take criticism well, and C. Ultimately I play mostly Lillith and certainly don’t have as much experience with Cass as you do so my instincts and experience might not be as relevant. In general your lists are very different from mine and I dissagree with a lot of your picks, but it may just be a playstyle difference.
And at its core a great player can get to S rank with a basic deck, the game is ultimately more skill then the deck building. And you certainly are skilled, likely more then me at least with abyss. But I have always prided my self in the deck building aspect, so I think I could help you optimize, or at the least a good discussion could help us both learn something.
Edit: ah Obliv Sphere is your current focus? Very interesting, it is actually midrange and does not have the coin flip issue nearly as much.