Redirect rules are a pain to figure out but not to hard once you understand the syntax. If I understand your goal then everything under 'https://www.mysite.com/fr/to
https://www.mysite.com/`.
Try this one, works for our scenarios but might not for yours.
<rule name="Redirect to Root" stopProcessing="true">
<match url="^fr(/?.*)$" />
<action type="Redirect" url="https://{HTTP_HOST}{R:1}" appendQueryString="true" redirectType="Found" />
</rule>
Note that you should set the redirect types as temporary until you have it working correctly. Once you set it to permanent then browsers don't have to go back to the original URL ever again which makes testing very difficult.