What is the best way

If you mean win an attack. Overkill. Lol.

Well the first way is to send spy drones I usually send around 10-20 drones, because the more drones you send the higher the possibility you get detected and even the higher the chance they being traced back. I usually operate with a scanning array lvl 8+.
Than another way is as you said to send an empty mk1 to the planet you’ll get a detailed report of all his ships and carries as well as all defence mechanism like shield and carrier bonuses. You should only do that right before you plan to attack the planet and also with a short travel time hence he can concentrate his ships on that planet. Another disadvantage would be he would know your identity.
Another way to spy just before attack is to send lots of spy drones with a success rate of 99% but keep in mind there is a high possibility they get detected and traced back, so you need to act fast.

Would you still get the stats if you told them not to engage, but sent them anyway?