1. Caching plugins can be good, but it depends on your hosting. I suggest running speed tests with and without to make sure they aren’t slowing your site down. I always prefer caching on the server-level.
a) The Read More text can be changed inside the WPSP list settings.
b) Go to “Appearance > Elements” and open the Custom Post Navigation element. In there, you’ll find the “Previous” and “Next” text.
c) In “Customize > Layout > Secondary Navigation” you can change the “Categories” text.
3. This is more difficult but possible. Is that the only additional rule you need (within the same category)?
4. I’m not sure what you mean here – can you explain a bit more?