Promoting child theme to stand alone

I have developed a website using a fully fledged theme packed with tons of bloatware and custom functions/widgets I am not using. The package is slow, and 95% of CSS (1MB!!!!) and Javascript is never used by the child theme.
This is why I am thinking about unlinking the child theme from the parent and promote the child theme as a standalone theme.
However this requires “walking” through PHP code, scripts and CSS to verify what is being used before trimming.

What steps would you take, generally speaking, to achieve this?

Start with “view source” of rendered HTML and then work backward from that, building a new theme by pulling whatever css, php files and functions from the parent as needed.

(footnote – thank you for the pointer to the ‘dust me’ utility, I will check that out).