It depends a lot on which race I'm playing, and whom I've encountered. I usually send a general message on first contact, then wait for a reply.
When I'm roleplaying a pacifistic race like my serene scientist Tiktsin in solo games, I don't even arm my ships until I discover We Are Not Alone, sometimes not even until we've actually been attacked.
I don't play pacifists against human opponents.

The honorable warrior Rrurrr try to maintain friendly relations and negotiate borders until they run out of expansion room or are attacked. The psychotic schemer Mi-Go try to maintain friendly relations as long as it serves their purposes, then focus on undermining their weakest opponent. I had a lot of success with the Mi-Go in their Last multiplayer game by picking up the scraps from the losers of other races' wars.
In solo games I normally play with "View All Scores", so I always know roughly how I compare to the AIs. In multiplayer, it's safe to assume I'm behind in tech and ships. I've been amazed at how fast my PBW opponents build up fleets and climb the tech tree.
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Cap'n Q
My first mod!
Hypermaze quadrant
The most merciful thing in the world, I think, is the inability of the
human mind to correlate all of its contents. We live on a placid
island of ignorance in the midst of black seas of infinity, and it was
not meant that we should go far. -- HP Lovecraft, "The Call of Cthulhu"