I think you have very limited options for a real upgrade from the U853s at $500 or less. Maybe Sennheiser MKE2 ominis or the Microphone Madness version of the same. The problem is that almost all of the mics that sound significantly better than the U853s (I'd classify the HLSC-1 and the Church Audio cards as lateral moves at best) require phantom power, which would mean a new preamp as well.
If I were you, I'd maybe get a set of omni or subcardioid capsules for the U853s that will give a more open, natural sound when acoustics are favorable and then save up until I had the $$$ for a phantom power solution and some significantly better mics like Neumann or MilLab actives or MAYBE Audix Micros. Other than the DPA 406xs, I can't think of stealthable alternatives that sound substantially better than the U853s until you move up into that price range.