I was surprised you reported this. I always had a problem like this earlier, so I did some experimenting and finally made a mod that changed some parameters that fixed this. Stardock updated the game to use these new parameters and eliminate the problem.
I then noticed that you are using 5 AI (and 6 total, I guess) in a gigantic galaxy. I mostly use a gigantic galaxy, but I always use the recommended number of races, which is me plus 15 AI. The work I did was using the recommended number of races.
I tried using the settings you listed and was surprised. The spacing was bad except when it was terrible.
I began experimenting with new parameters using only 6 total players and your other settings. I would start a game, then use the console to look at the whole galaxy to check the spacing. I found new parameters that work with your settings. The races are widely, but not evenly, scattered. There is a tendency for 4 or 5 to start close to the edges of the galaxy with 1 or 2 in the middle two-thirds of the galaxy. Sometimes they are evenly scattered with only 1 or 2 near the edges. I haven't been able to do better than this. Sometimes they are closer together than other times, but I never had any appear too close together.
I tried using these new parameters with my normal galaxy settings and found the spacing of the races was bad, so they only work when the number of races used is much less than the recommended amount.
I have made a mod that uses the new parameters. You can get it here.
Unzip it into you mods directory and enable mods. All new games should have much better spacing of the race starts.