why he stepped down as CEO, then CTO and now IC?

If I had to guess:

1. Perhaps because he can.

2. Because being a CEO of a public company comes with a lot of rules around disclosure of material public information and equal access. It takes a special kind of person to disregard general consel and just shitpost on twitter with zero review while directly responsible to shareholders. I don't know what kind of safe harbor Elon thinks Twitter offers but I doubt it applies to Github code review.

If I was to guess, he wants to write code and he started the company so he could write code. He just had to do those jobs along the way to get to the point where he could just focus on writing code and solving problems.

Yes that's exactly the reason. I know them through friends

CEO of a publicly traded company is a vastly different job than CEO of a private startup. Your job is to make money for shareholders, not pursue a vision. It's not something everyone wants to do and is likely a lot less rewarding for someone who successfully creates a technology company.

So the company can bring in outside execs without ousting one of the founders from a C-suite role.

He's a true engineer at heart. This is him at Dev Tool Time recently: https://srcgr.ph/mitchell-hashimoto

He has enough money now and wants to do what he loves.