In order, Earth has the misfortune to encounter:
* Circa 2190, the Gamilons, located in the Large Magellanic Cloud.
* (Plus, in 2199, Beemera/Beeland and Iscandar.)
* 2200, the White Comet Empire, arriving from outside our galaxy.
* (Plus the recently dead civilization of Telezart.)
* 2201, the Black Nebula Empire ("The New Voyage" and "Be Forever Yamato"), from the Virgo Cluster.
* 2203, the Galmans and the Bolar Federation.
* (Plus the not-so-recently mostly-dead Shalbart/Gardiana and the intelligent planet Phantom.)
* 2205, the Dinguil ("Final Yamato") and the intelligent planet Aquarius.
* 2225, the Great Urup Interstellar Alliance, consisting of 10 powers including the Amare, Beidel, Ethos, Fridei and SUS.
No, there's nothing odd about this if you remember the galaxy is 100,000 light-years wide and contains 400 billion stars ... except maps were shown in the early stories that showed the Galmans and Bolar pretty much carved the place up. (At least ISTR as much. Can anyone corroborate my recall?) Where did everybody else squeeze? Unless the maps were over-reaching by those powers; they claimed the areas, but hadn't properly explored them. (A lot of visual SF doesn't properly acknowledge how big a galaxy is. Travel a thousand light-years by lunch? There's nowhere to hide!)
More importantly IMHO, there's a dramatic problem. "Ho-hum, we disposed of this adversary in one story. What? There's demand for a sequel? We'll have to create a bigger and badder enemy, with a completely new ship aesthetic." Dessler got several encores, and the Black Nebula Empire appeared in successive movies, but that's it for recycling. By contrast, in Star Wars, the Timothy Zahn etc. novels had remnants of the First Galactic Empire -- plenty of leftover Grand Moffs and Grand Admirals to scheme against the heroes.
(My dates are probably skewed. How is it that, among all the production timelines on OurStarblazers.com, there isn't one for in-camera continuity?) See also the three "History of the Star Blazers Galaxy" articles, per the work of Bruce Lewis in the mid-1990s.