I've discovered a range of superb decals for First and Second World War aircraft in 1/285th - 1/300th scale, from a company called Moorland Scale Model Decals here in the UK. These are resized from individual decal sets in larger scales, so have excellent detail and accuracy. The only downside is that the sheets are not based around specific RFC Squadrons or Jagdstaffel but are for individual pilots, with an emphasis on well-known aces like Billy Bishop or Carl Degelow, for example. I'm thinking of asking Geoff to design some for Jasta 18 but will hang on for the moment and see if Dan Arute at Flight Deck Decals can design a whole sheet of insignia, as he is my 'go to' for decals. They do look fabulous though, and I'm sorely tempted to order the SE5a sheet just for kicks!
https://moorlandscalemodeldecals.co.uk/285th-300th-scale-sets/