The latest robots.txt included in Joomla goes in that direction :

User-agent: *
Disallow: /administrator/
Disallow: /bin/
Disallow: /cache/
Disallow: /cli/
Disallow: /components/
Disallow: /includes/
Disallow: /installation/
Disallow: /language/
Disallow: /layouts/
Disallow: /libraries/
Disallow: /logs/
Disallow: /modules/
Disallow: /plugins/
Disallow: /tmp/

The /media/ folder is removed but you can still find js/css files (not to mention images) in /cache/, /plugins/, /modules/ or /components/.

A quick and dirty way to solve this is to remove these directories.
Then you can add your lines to restrict access to js/css/images files. BUT, you still need to specify some Disallow, unless you don't care about restricting anything. But remember, what googlebot can access can be indexed, and you may not always want that.

Before I ran into your solution, I used to be very specific, so I have robots.txt looking like this:

User-agent: *
Disallow: /administrator/
Disallow: /cache/
Disallow: /cli/
Disallow: /components/
Disallow: /includes/
Disallow: /installation/
Disallow: /language/
Disallow: /libraries/
Disallow: /logs/
Disallow: /modules/
Disallow: /plugins/
Disallow: /tmp/

User-agent: googlebot
Disallow: /administrator/
Disallow: /cache/
Disallow: /cli/
Disallow: /components/
Disallow: /includes/
Disallow: /installation/
Disallow: /language/
Disallow: /libraries/
Disallow: /logs/
Disallow: /modules/
Disallow: /plugins/
Disallow: /tmp/
Allow: /plugins/system/jch_optimize/assets2/
Allow: /components/com_fabrik/views/tmpl/

I use the list of blocked ressources from the Google Search Console to customize the file. it's more work but gives you the most control.