Through hole SMA parasitic effects
Assuming that the connector is fine for 6 GHz, the 3mm or so protrusion on the solder side would become an issue if 3mm represented about one-tenth of the wavelength of 6 GHz. This is a general rule of thumb and some people say one-twentieth but I'll stick with 1/10.

6 GHz has a wavelength of 5 cm and one tenth of this is 5 mm so, you could argue that it's beginning to be a small effect. Maybe just trim it back so that it barely protrudes.

If it were a quarter of a wavelength long then you would have effectively an impedance transformer that would look like zero ohms and that wouldn't be very good but, a quarter wave is 12.5 mm and you maybe have 3mm protruding (PCB thickness dependent).