I decided to catch up on Moon Knight after not paying attention to him for 40 years. So I did some digging and came up with some Youtube videos. Casually Comics looks at his first two appearances, while Comic Tropes looks at his entire career.
This confirms my memory of his early history, which is that at the very start in the 70s he had no relationship to an Egyptian god or explicit mental issues. Instead he had multiple civilian aliases/identities he used for convenience while fighting evil-doers - a rich playboy, a cabbie, and a mercenary (the last being his real identity.) He also had a girlfriend who knew about his aliases and Moon Knight.
The YouTubers above, being younger and familiar with the modern character, are a bit baffled by this multiple-aliases-but-no-mental-issues incarnation of the character.
Back in the day, though, young nerd me knew the source of this version. The first version of Moon Knight was based, not on Batman per se, but the Shadow, in particular the pulp (not radio) version. The Shadow, in his civilian life, generally pretended to be rich playboy Lamont Cranston. But he used multiple other civilian identities and aliases as well when investigating crimes. And he was really aviator Kent Allard, not Lamont Cranston at all. Just like Moon Knight, the Shadow had a girlfriend who knew all about his other identities.
Much later (later even than connecting Moon Knight to an Eygptian god) writers decided to make these multiple aliases not mere disguises he adopted but actual multiple personalities. Thus modern Moon Knight was born.
(The Shadow, of course, was a major influence on the creation of Batman. So one way to look at it is that Moon Knight isn’t Marvel’s Batman: Moon Knight and Batman are both Shadow rip offs instead.)