I might visit with the client about using Woo. It does a lot with the free versoin, and makes it the easiest way I’ve found to collect payment.
But otherwise, I do think a child theme would be the best way to do this. You can simply copy the appropriate theme base files like tag.php, categoryphp, index.php (or patet-posts.php), any file that emits “Post” or a Post related title, copy them to the child theme, and simply edit the files to say whatever you need. Don’t really need to know PHP or anything – just search for the text you need to change.
You might have to keep up with changes to the theme, but usually those files remain unchanged over many mnny releases as they are pretty basic.
A totally different alternative with text strings is to create a custom translation setup, and change the strings by translating to a different string. But that then requires using a translation editor as well as a child theme where the language files are changed instead of the php files.