This is tough. I don’t know of an automated way of doing this because every theme is different and who knows what changes you (or whoever) have already made to the original theme.
In my opinion, if you want to do this correctly and with the best results you are going to need to do this manually.
I would, setup a local install of wordpress, download the original unedited theme and install it on my local setup, then make your child theme. You could go through the theme options in the admin fairly easy just by opening your local site and the live site side by side and comparing. This will get all your settings aligned. You could also grab any styles from the customizer and put in your local site.
With the site settings aligned, now you need to make the same edits to your files but this time within your new child theme. Depending on what IDE or text editor you use, you can probably install a diff tool that will compare the differences between files. Similar to this online diff tool.
If you go slow and go section by section or page by page and compare the live version with your local install eventually you will have a matching child theme.
Not fast but it will get you there!