Add link function won't include pages/posts when searching

OK

And everybody know how easy it is to export a page which has been created in the Block editor :(

What is the issue there ? With exporting a page ?

Only export selected pages and get media these pages include.

Anyhow, not easy to know if there is legacy code in this case.
Will your suggestion be creating a new blank site and import pages/posts?

Yeah i would start with a new install and import the posts.

Try to find legacy code will not be easy.

Unfortunately there is not much else i can advise.
You may want to ask in the general WordPress.org forums as more knowledgeable people then me may have some insight to this.

Thanks for your help.

You're welcome - sorry i couldn't be more helpful