>it would also be a demonstration of development and access to resources beyond our imagination. That's the upside [...] In a galaxy of sparse and sparingly advanced civilizations, the message might be "fear us and stay away" in a way that EM would not convey.

I think you've hit the crux the question. If there are only a few civilizations, I agree, that'd be an awe-inspiring deterrent.

However, if you don't know how many civilizations there are that are similarly advanced to your own, sending out a big "we're here!" message may be quite risky.

In terms of game theory, it's a sequential and incomplete information game. I think the smartest decision is to remain quiet.