A signal travelling slightly ahead of the cavalry would be like a trumpet call announcing "here come the nanobots!", giving the adversary time to prepare.

Yes, but we better prepare for nanobots anyway. If they don''t come it's just a bonus. It is wise to be prepared for an intergalactic war in any case. For the robots, for the small kinetic projectiles with a near light speed, for some artificial gamma ray bursts, for SETI attacks and for many more.

Then we should strike in all directions in the best tradition of a very benevolent colonist. To end all the space wars even before they relay start. As much as we can.

The aliens, who are extremely rare (as I think), had, have or will have the same dilemma, what may be another opportunity. Game theoretically speaking, we must do some calculations right now, it is already late and the OP's article is a good one.