For finding left over code, you’ll probably want to hire a developer, if you aren’t familiar with coding.
One thing I found on the front end is that you might want to remove links to buddha-crosssell-db.js from your code as that is resulting in a 404 error.
There’s also an error in your theme.js file you might want to look at/ hire someone to look at.
As for improvements, they’re pretty much the standards for most shopify themes:
Make resources non render blocking
Preload hero image
Remove lazyloading from hero image
Preconnect to critical origins
They’re are multiple versions of jquery being loaded (3)
Remove src link from lazyloaded images, leaving only data-src
Optimize images with a compression app
Consider deferring the loading of your tidio chat app to scroll.