Looked at the preamble and was rather excited about play fair which this turnout not to be. Good and challenging grid construction but nothing too taxing in the puzzle. But I have that nagging feeling that I may have missed something as all the playfair stuff seemed a bit over the top given my solution, so maybe my first sentence is wrong. Going to be mulling this one for a bit...
Yes, AndrewG-S, I'm in exactly that position. It was a very easy grid fill. When I made my first letter swap (I was doing them in order) I thought 'Oh, this is going to be very clever.' But that sense of excitement faded with the remaining swaps, so I too am thinking there's another layer in there somewhere.