How often do new servers start? and after what time would you say it is no longer really safe to join a game after it has begun?
For now servers typically last 5-7 months. On the Main2 server there is a “chance” that joining might put you beside more veteran players, though it’s designed not to. With the ability to travel distance there to find planets that can be colonized, it can happen.
On awaken the dark (the fast server), you don’t spawn into existing planets that could have previously been colonized by others, so it’s likely you’ll spawn in by people who are no more veteran to the game than you are.
Also if you do spawn somewhere occupied by veterans, you can always reset and try again to land in a more friendly environment.
Its probably the other way round, if you wait with spawning you will be further away from the core of the galaxy and therefor much less likely to run into veterans or warmongers. just join now the awaken the dark server and have fun!