There is a hidden rank above SSS+, this, while impossible for mortals, is not impossible in with the help of a god, a god can bless someone and mold them into an heir for their divine role, the rank above SSS+, that is only ever possible if a god chooses you, is rank EX. This is when you stop being mortal and gain divinity, you gain a concept, this is what makes breaking that ceiling so hard as well, you need to choose a concept and become aligned with it, a very domain of reality, only then can you cross the threshold, it isn't about getting stronger, combat will mean nothing at that point. Though only the gods know of this. Basically think of it like this, the difference between SSS+(which is the natural limit of mortals) and EX rank is not strength-based. It's identity-based.
You don't break into EX by grinding levels. You do it by aligning with a fundamental domain of existence, and only a god can initiate that transformation. Also important to note is that Gods can intuitively sense soul rank potential, so they keep an eye on anyone born with an SSS+ rank potential, since only those people could potentially be qualified to be chosen, and their innate ability is what determines which gods are interested, like for example, someone with a light related innate ability and SSS+ rank soul potential isn't going to interest Nyx, it would interest Eos though. And someone with an SSS+ rank soul rank potential but an innate ability related to death would attract Hades. Also for the gods, there are different pantheons, the primary pantheons though are going to be Greek, Norse, Egyptian, Hindu, Biblical, and Mesopotamian, though the biblical guys(aka angels and abrahamic god(Yahweh is not omnipotent in my world, he is about god-kind level, same as Odin and Zeus). Though the gods won't become very important for a long while.
Also important to note that the only way to get to EX rank is to gain a spark of divinity, you then become a demigod. The spark of divinity needs to be refined by deepening your understanding of the concept you've chosen, it isn't about getting stronger, you have to deepen understanding and align with the concept to raise your tier. The tiers are as follows:
1. Demigod(aka EX-Rank)
2. Tier-1 God
3. Tier-2 God
4. Tier-3 God
5. Tier-4 God
6. Tier-5 God
7. Tier-6 God
8. Tier-7 God
9. Tier-8 God
10. Tier-9 God
11. Tier-10 God
Important to note, God-king(Zeus, Odin, etc...) is typically around tier-8 and tier-9, and Primordial is Tier-10(aka Nyx, Chronos, etc...). The jump from tier-9 to tier-10 is massive, it requires aligning so deep with your concept that you become conceptual, for example, you don't just 'use' fire anymore, you 'become' fire itself.
Also, once you reach EX rank(demigod), the persons mana core changes to a divine core, you stop using mana and instead use divine energy(divine energy being a higher form of mana and is fully unlocked once you hit Tier-1 God, only partially available at demigod rank. Also once you reach EX rank(demigod), your soul evolves with the spark of divinity, granting partial access to divine energy and the divine core.
Also blessing someone with a spark is spiritually costly for the god — a kind of divine investment. This explains why gods only do it once per era per god, they wait until they find the right person to be their heir.