get_template_parts() usage

Yes, you are using the function wrong. You don’t need to echo the return of get_template_part() – the template it references will automatically be echo-ed.

Furthermore, you don’t want to have a leading forward slash (/) at the start of the first parameter, or the .php extension, nor the second parameter.
Your function call ends up looking for:

my-child-theme//template-parts/hero.php-hero.php
               ↑                     ↑    ↑
          Double slash caused        │  Extra -hero from the second parameter.
          by leading / in first      │
          parameter              Extra .php
                                 from the first
                                 parameter

Rather, you’d want:

// my-child-theme/template-parts/hero.php
get_template_part( 'template-parts/hero' );