Sorry for the slow reply.
I think you can get away with a force fully qualified URLs plugin on the site, if they’re OK to try that.
I removed relative URLs options altogether from recent versions, as it can cause more trouble than worth. FQURLs always safer.
In this case, are you sure that WP2Static is the one converting your links to relative? If unchecked, it shouldn’t be, so they may be coming from somewhere else.
I’ll need more information, usually a good start is a sample of the source HTML code from your dev WP site, then how it’s turning out in the statically rendered HTML. This will confirm how, if at all, the plugin is changing it.
From 6.6.7, you’d probably want to move to Static HTML Output, which is very similar, but with a lot of bugs fixed.
If you can easily export the whole site, you can try importing it on your local machine with Lokl and doing test exports from there to some temporary static sites, ie, free ones on Netlify.