You can remove the [Reply] button from first post using this CSS code in Dashboard > Forums > Settings > Styles > Custom CSS code textarea. Save it delete all caches, navigate to forum front-end and press Ctrl+F5 to delete browser cache:

#wpforo #wpforo-wrap .wpfl-4 .wpfp-first .wpf-content-foot .wpf-reply {
     display: none;
}

Then you can Stick the first post on all pages of topic or set a big number of posts per page in Dashboard > Forums > Settings > Topics & Posts admin page. More info in the documentation: https://wpforo.com/docs/root/categories-and-forums/forum-layouts/threaded-layout/#thread-managing-options