Ranking shouldn’t be affected as long as the menus stay the same. But you shouldn’t base your success on traffic, but on sales or conversions. If your net sales are 3k a month, it’s better to hire someone on Upwork for a new theme design. Don’t use Themeforest because they often add extra code that slows down your site.
I think you should find someone who knows Divi or Bricks Builder.