What an intriguing design! ! ! !
For a model of that size span and area there's no point in going crazy with fancy options. The small wing chord will make shaping of a really critical airfoil too fussy to bother with. I'd say you want SOME amount of camber but not a flat bottom section. Make something that looks more or less like a NACA 2410 and call it a day. Or you could use a Randi Randolph style airfoil of 10% thick and it would be fine. What's a Randi Randolph style airfoil you ask? Randi had a number of his RC sport flying designs published over the 20 year span from around the late 1970's up near the turn of the millennium. Some used a fairly flat bottom shape and others used a low camber nearly symmetrical shape with a flat rear portion to sort of simulate an airfoil which would be much like a NACA 2410 but with a flat lower rear portion. Although if you look at that link above it would not take a lot to flatten the rear underside half of the shape. Either way the flat shape allows you to build the wing easily on a flat building board. And at this size it's as good a shape as any.
A primary concern is going to be to keep it light. The aspect ratio is high so there's not a lot of wing area. On my screen the top view has a 90mm span and a 10mm chord at around where I'd expect the MAC to be. Scaling that up to 1200mm means a 133mm chord. And that's about 50 to 70 mm smaller than most sport model wing chords for that sort of span. That's going to cut into the wing area so making the model lighter than the span would suggest is going to be pretty important if you want a friendly sort of landing and takeoff speed. I know you want retractable gear but keep in mind that such gear assemblies and the hard points in the airframe to allow using them can add up to a lot of that weight. You may want to consider a takeoff dolly and belly landing in the grass instead. The in flight performance for both climb and even speed would be better.