Moving files to the SD card often doesn't do that much to free up Internal Storage, because documents and media files typically don't take up that much space (unless you're talking about big movie files or tons of high resolution photos). What takes up most of the Internal Storage will be apps, and if you are able to move an app to the SD card, that's typically just a portion of the app. The main part of the app still needs to remain on Internal Storage.
The bottom line is that you either have to uninstall apps that you don't use, or upgrade to a phone that has much more Internal Storage. 64 GB is pretty much the bare minimum these days.