I know it's difficult to pick a suitable TEFL/TESOL course with places all too willing to puff up the aftermath of their courses... But you have a Masters degree and I would guess that the route you took towards your Masters degree had some actual classroom teaching, so I reckon that would be transferable to teaching Adults.

And maybe when you take a break from your full time work, perhaps you could ask a local language school if you could join their staff for a week or so.