Pretty links with add_rewrite_rule and add_query_var

There are a couple of issues with your regular expression:

^model_more/(\d*)$
^--- Says "match start of line"

In your URLs, the start of the line is actually /model/, so ^model_more will never match. Instead, try a regex like this:

^model/[^/]+/model_more/(\d+)/?$

• [^/]+ says “match anything except /, since I would guess there are multiple models under /model/.
• (\d+) ensures that only digits are captured – (\d*) would capture anything after the number.
• /?$ says “the end of the line might be preceded by a slash, but not necessarily.

Edit: You can see the regex in action at https://regex101.com/r/OhOFTD/1