“read more” tag for excerpt() won’t show full content

get_the_permalink() does not modify or do anything to the_excerpt(). That’s now how viewing post content works. All it does is output the permalink the single post.

When you visit that link it will load a single post using the template it finds according to the template hierarchy. You need to make sure that the template that your theme uses for the single post uses the the_content() function, instead of the_excerpt().