You can combine and minify CSS and JavaScript files using tools that are specially made for it such as WP Rocket that we use for our website as well.
Loading assets from a subdomain won’t give a thing to performance. What you should do is to use CDN such as Cloudflare, KeyCDN or other. Using cdn you will be able to load assets from subdomain such as cdn.yoursite.com or use URLs provided by CDN itself.
There are many resources to speed up your site. You can use W3 Total Cache. It’s fairly complicated and can break your site. There’s a guide that explains all part of the plugin and how to use it best for your site. You can also make use of a really good image optimizer such as ShortPixel, WP Smush Pro, Imagify etc.
Autoptimize is fine too. Obviously, WP Rocket will provide additional benefits and will make your website faster but, like everything, comes with a price.