Unable to display selected post title in frontend from metabox wp_dropdown_pages()

That’s because, as documented the value of options in wp_dropdown_pages() is the ID:

‘value_field’
(string) Post field used to populate the ‘value’ attribute of the option elements. Accepts any valid post field. Default ‘ID’.

So since you’ve saved the ID, use that to get the title:

echo get_the_title( $related_workshop );