Well you said: " In fact, the natural level 5 might be more similar to a serum level 5 than the big three" but that is a completely nonsensical statement because the big three are the ONLY ones who achieved a natural 5th evolution...you can't separate the two...a natural 5th evolution cannot be anything like a serum 5th evolution. We are even told that there are extreme dangers to attempting a natural 5th evolution...which most likely has to do with controlling or bonding with whatever entity like the big 3 did, which is where their extreme power comes from.
What I was saying is that a normal natural level 5 might be very different to what the big three are. If a natural level 5 is to be claimed by an apostle, it makes no sense that they'd separate from them, or whatever actually happens.
Henri, a natural level 5, and possible inheritor to Paradox, is rejecting his origin. Berny and Malik, H.E.R.O's only other documentation on natural level 5s, achieved theirs in the same way.
Meanwhile, a level 5's purpose, according to Ella, is to signal to the apostles that the superhuman is "ready". This is contradictory, hence why I think that Henri, Berny, and Malik are not normal level 5s, and that since they're H.E.R.O's only example of a natural level 5's, they think this is normal.
Which, by the way, we know is false. Eisheth considered Berny to be something that shouldn't exist, so I doubt someone like that would be the result of the normal, intended evolution procedure.
Meanwhile, an artificial level 5, like Ella, is still capable of being a Chosen, so at the very least, she's likely capable of being claimed as a vessel, at least eventually.
An alternative thought that I had, is that a superhuman isn't supposed to naturally reach level 5, because they should monsterfy and become the apostle's chosen during or before that. By supressing that, you sever connection with the apostle, and become what the big three are. Meanwhile, artificial level 5's are skipping both the monsterfication and the suppression, going into an awkward stage that isn't supposed to exist.