How to change content hash value, within the_block_template_skip_link action?

If remove_action( 'wp_footer', 'the_block_template_skip_link' ) did not work, then try with remove_action( 'wp_enqueue_scripts', 'wp_enqueue_block_template_skip_link' );, or both of that. (see source on GitHub)

As for changing the href value, I’m not aware of any (filter) hook to do that, but you can either edit your template and set the <main>‘s id value to content-top, or you can use JavaScript to modify the href value. E.g.

document.querySelector( 'a.skip-link[href="#content"]' ).href="#content-top";

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