First and Foremost! Android 6 DOES allow SD card to be formatted as internal storage. Like if your internal memory is 16 gb and SD card is 32 gb. As soon as you insert it into the phone, you will get an option to format it as internal storage boosting it to 16+32 gb in total.
BUT
Sony removed this feature in xperia hence you can only use SD card for pics, vids and files.
Here are some things you can try.
1) Perform a factory reset of the phone and dont connect it to WiFi/Mobile data. Bcoz it will automatically update pre-installed apps and again causing more loss of memory. You can disable apps like what's new, track id, google play movies, etc.. all of those which you wont use.
2) Install the lite versions of facebook/skype. They take less space.
3) In Camera app settings. the storage should be selected as SD memory instead of internal storage, so everythime you record video or take a pic, it is saved directly in SD card.
4) Install minimal apps and dont install apps which show more ads. Because these applications which show ads downloads ads to your internal memory in background.
Hopefully sony gives the extending feature in Android 7 Nougat update. fingers crossed
