I believe that Broly is clearly a galaxy buster, my reasons are.....
1. Planet Vegeta was in the North Galaxy when Broly escaped as a baby, there was no way for him to traversed thousands light years to the south, this means that he was destroying planets in the North and not the South.
2. He was toying with all the Sayians, Plus he quickly eliminated each Sayian from the fight with mere punches, blasts, and hits. Kid Goku was knocked after a single blow from Broly, this goes for Vegeta. Same with Trunks and Goku also Piccolo.
3. The movie start was not a fast forwarding, it is the south being destroyed. King Kai reacted quickly and told Goku this, It wouldn't make sense if the south galaxy was destroyed and king kai didn't told Goku till he was older.
4. Broly was shown to be the most destructive character in DBZ with the most destructible power in all DBZ though this does not make him the strongest.
This is proof of why Broly is a galaxy buster. Search it up if you want, Planet Vegeta was located in the North and not the south and it was never stated that Broly as a child went to the south, he only went to the nearest planet and started to destroy planets in the north. Paragus must've took Broly to the south. Also destroying planets are not a big issue in destroying a galaxy, the stars are what makes a galaxy.