Welcome to Android Central! Deleting cache is very temporary, since your apps will start building up the cache again right afterwards. You can clear all app caches by going to Settings>Storage and tapping Cached Data.
I would avoid any cleaner apps, since many of them do sneaky or shady things (like installing adware, or including hidden "features" that usually involve ads as well). If your Internal Storage is that full, you'll just have to start uninstalling apps, assuming you've already moved all media files and documents to the SD card.