Have you actually played CoC? It sounds like you haven't.
You can't start with any points in Cthulhu Mythos. At all. You don't gain skill points by using Cthulhu Mythos. At all.
Let me quote DIRECTLY from CoC for you:
"Instead, points in Cthulhu Mythos are gained by encounters with the Mythos that result in insanity, by insane insights into the true nature of the universe, and by reading forbidden books and other Mythos writings. A character’s Sanity may never be higher than 99 minus his or her Cthulhu Mythos skill. As Cthulhu Mythos points proliferate, they crowd out Sanity points, and leave the investigator vulnerable."
It exactly works like I'm suggesting. EXACTLY. You're flatly and completely wrong. Your Mythos skill is acts a hard barrier to your max sanity. This is not how the suggested system in D&D works. It is how the SAN/ALN system I proposed works, though, but you ignored that in favour of the outright false claim that Cthulhu RPGs let you be sane and have high Mythos. They do not.