How to Remove HTML Elements from Post Excerpt?

At first glance, this appears to be an XY Problem.

If you simply want to increase the length of the excerpt auto-generated from the post content, you can use the excerpt_length hook in functions.php.

function modify_excerpt_length( $length ) {
        return 300;
}
add_filter( 'excerpt_length', 'modify_excerpt_length', 99 );

Then simply call the excerpt in your template:

the_excerpt()

This will automatically remove all tags.