Sometimes a thick airfoil trailing edge can even reduce drag - witness US Patent 4858852A by McDonnell Douglas Corp. However, this applies to a supercritical airfoil flying at much higher Reynolds and (most likely) Mach numbers than what you have in mind.
A similar experience was already made 40 years earlier by Northrop when they could improve the flight stability of the Northrop X- Bantam by stabilizing oscillating shocks by thickening the trailing edge. The X-4 used double split flaps which opened to both sides, and by glueing a strip of balsa wood between them to keep them permanently opened at 5°, its pitch problem onset could be shifted from Mach 0.88 to Mach 0.92.
But that will not help you much if you do not intend to fly at transsonic speeds. However, a thick trailing edge is generally preferable to a razor-sharp one because it will stay straight for longer and be less of a hazard to ground crews. Since at the trailing edge you will find a several millimeters thick band of slow-moving air from the boundary layer, adding 0.5 mm from a thick trailing edge is not increasing drag significantly.
If you want to simulate the effect of a thick trailing edge: Mark Drela's XFOIL software allows to prescribe trailing edge thickness as a percentage of airfoil chord. It just so happens that XFOIL is ideally suited to study subsonic flow at the Reynolds number range you have in mind. I strongly suggest you try it out for yourself.