The drop might be related to technical SEO issues introduced during the migration. Things like changes in URL structure, loss of meta tags, increased page load times, or inadvertently blocking crawlers could be the culprits.
Before jumping ship to WordPress or reverting to HTML, I'd recommend conducting a thorough audit of your Drupal site. Check for common migration issues: verify redirects, review robot.txt, ensure all meta data is intact, and monitor page speed. Drupal to WordPress won’t magically fix ranking issues if underlying technical SEO problems aren't addressed.
In my experience, post-migration drops are often temporary and recoverable with the right fixes.