I know it's controversial to do things this way, but given it's a personal project and focuses on portability, I think the benefits are worth while.
I have taken care to break things up, comment, etc - if you start at main(), you may find it's not any more difficult to follow than if i'd broken it up. Just because it's one file doesn't mean it doesn't separate concerns.
>"Contributions are very welcomed. Please feel free to submit proposals directly in the form of a PR or Issue."
Good luck getting contributions. I don't have the time to read and understand all 4000+ lines of this very long file. I have ideas, but I'm not even interested in participating because of the format.
I have taken care to break things up, comment, etc - if you start at main(), you may find it's not any more difficult to follow than if i'd broken it up. Just because it's one file doesn't mean it doesn't separate concerns.