The shop page is the functional page and it cannot be changed. You can create a custom page, and use the Product Grid element and other elements to show, then you can show the page as a shop page on the menu. At the same time, the real shop page is on your site but not shown.
Another approach: As it is possible to place the “cateogory-text” above and below the product-grid, it may be possible to address both containers? Like a teaser-text above, and a long text (needed for seo) below?
The additional content can be placed either before the product grid on the product category or after. You can configure the option in the Theme Settings > Shop > find the Category description position option. However, it is not possible to insert the content both in the top and in the bottom