What speed chain are you using? From scanning the web on this 10 vs 11sp issue, I found out that the outside width of 11sp chains is 5.4mm and of 10sp is 5.9mm. With the bikegang spacer one might fear that 10sp chain could be too wide. I ride with 10sp.

If the 16T cog was too loose for you, you might try putting back the Brompton chain guide. In my packing of pieces on the cassette I had to take the guide off and this seems to be most common when increasing the cog number, also in your photo. However, I eventually managed to put the guide back, this to cure the chain sometimes getting caught between the largest cog and hub for folded bike. I accomplished the feat by removing the most inner part of the guide and then placing the guide loose between the largest cog and the hub. Over a couple of days the guide occasionally pushed the chain off the largest cog, but then apparently enough of the protruding plastic rubbed off and the rear gears went back to complete normal.