Most secure sd card setup MOTO G 3rd with Android 6.0 (Encryption / adopable etc)

[smib123]

Hi, I have a MOTO g 3rd gen 16gb and I have a 32gb micro sd. I'm wondering the most secure set up. I want all the data on my phone and sd card to be encrypted. I know that using the sd card as portable memory leaves the data on the sd card unencryped so that is a no go. At the moment I am using the sd card as internal memory which with my understanding would mean that all data on the phone and sd card would be encrypted. However when I connect my phone to the PC via USB I can explore the files on the SD card, including photos music etc. If my understanding is correct then I shouldn't be able to view this as it should be encrypted? What is to stop someone stealing my phone and connecting it to a computer and getting my data? Any help you could give me to help my understanding/problem would be great.
Thanks in advance