This is exactly what I did. I'm glad I started reimplementing everything when I was younger because I might not have had the patience for the grind. But after 7 years of that, I have enough momentum that it's easier and easier. It also helps to have good architectural principles. I don't treat my hobby projects like throwaways because that's how you end up with frustration.

The end result is being completely unaffected by version changes, the trends in programming that go up and down (javascript, python, whatever, doesn't matter to me what's happening in their world).

It helps that I was also an early adopter of Rust from pre-1.0 in college around 2011, and so I've had a relatively stable implementation language. That combined with C and Luajit is my permanent stack right now, but I'm in the progress of migrating to my own programming language.

The natural terminus of the path you are suggesting is to reimplement everything. Be prepared for a long road, even with LLMs at your disposal now. An LLM can't design what you want because it can't read your mind.