That doesn't necessarily mean that he was conscious right after he was shot. The shooter could have thought he was dead. Seth could have regained consciousness later after the shooter left. I have no dog in this argument and have no idea what happened to Seth Rich. But one can not rule out a pro hit. More facts would be needed.
Does anyone know the caliber of the weapon used?
You are right about that - nobody knows anything. Everything is speculation. We don’t even know if it is being investigated or not. Hopefully, no one on this forum knows what a professional hit is supposed to look like. To speculate on whether it was or was not is just that - speculation.