As I said, I don't think there will be any Akuma past Level 4.
You see, number 4 in Japan is
associated with death. Even numbers which
contain number 4 are generally unlucky-sounding. Much like number 13 in America, there are often no floors or hospitals with number 4 in Japan.
And what do the Akuma symbolize?
Death.
Even in the Western Christian mythology, you have Four Horsemen of the Apocalypse. And what is the purpose of the Akuma? To bring fourth the Apocalypse.
Considering that D.Gray Man is a highly symbolic and mythological work*, I simply do not see Hoshino ignoring this symbolism.
Also, your Bible symbolism is wrong.
Number 6 is not the Devil's Number, it is human number. Number 6 also represents imperfection and incompleteness, because humans bear the Original Sin and have thus fallen out of the divine. Man was created on the sixth day; on the seventh day was creation of the world completed.
Number 666 is the Number of the Beast because it represents rejection of the divine in favor of human under influence of the Devil. It is symbolic because number 7 represents the divinity and perfection, and because there are three divine persons (the Holy Trinity) and 3 is also divine number. So triple repetition of 6 (
it is not 666 but 6 - 6 - 6) represents the "
perfect rejection of perfection", or else the human quest for divine which is what got them thrown out of the Eden in the first place (the Original Sin is a consequence of Adam and Eve seeking to become akin to God, or even become Gods). And Devil himself had also rejected God while seeking divine. Three sixes also parody the trinity (three sevens).
* Examples:
The Noah family: Millenium Earl and 12 disciples.
Literally the Antichrist and his 12 Apostles.
The Innocence: There are 109 pieces of Innocence. Miranda's Rewinding Town kept resetting to October 9 or 10/9. And if you exclude the Cube, this leaves 108 pieces of Innocence - which is an important symbol in Buddhism:
108 earthly temptations (Bonnō) a person must overcome to achieve nirvana.
Allen Walker's birthday is on 12/25. Or 25 December.
I don't think I need to explain that one.
Anyway:
https://fantasyview.wordpress.com/2023/11/01/allen-walker-is-the-best-protagonist-in-manga/