That's a nice looking bike, congratulations! How do you like it so far? Also that is great info about the actual seatpost insertion depth! That is interesting you are running a 30.9mm with a shim, rather than the 31.6mm. Depending on if the restriction at the bottom of the tube is a hard stop, or a sort of slight narrowing, running a smaller diameter post could be a way to get a bit more insertion depth out of the frame.
Not that 5mm is likely to make a big difference, but if the Medium Force allows one to fully slam a 180mm OneUp, then it seems the Flux is more restricted, which seems like an odd choice for Teewing to make when designing their 2nd bike, especially one intended for even more aggressive riding than their first bike.