I have a $70 Blu phone and I've run out of space a lot. Our friends at Google like to push updates on me, and there's a bunch of crapware that I never use but can't delete.
I presume you've gone to the Settings--> Device--> Storage -->set the default storage to the SD card.
You can also try going to settings--> Device-->Apps and go through each app. Some have a button to move to the SD card. Disable ones you don't use, uninstall them if you can, clear the cache (some say "manage space"). The next step is to uninstall updates. Sometimes you will get a lot of space back. They seem to keep all the old updates just in case you want to use them ....?
Anyway, the next thing to do is reset your phone completely. You can probably get back half your internal storage, because that's what it came with. Google will immediately start filling it up again, but at least you're starting fresh. Kind of drastic, because you'll have to fiddle with it to get back to the way you like it.