@Stuart ********
internship in relation to education is allowed on a non ED visa, and without the need for a work permit.
Doesn't matter if it's paid or not.
Did it myself for 6 months 2 yeas ago, finishing a bachelor's degree here in Thailand.
Got documents from my university and workplace where I did the internship, and it was checked by several officials in both the labor office and immigration. All okayed. Had zero issues with that.