Not an artist here, but dounderstand some about Manga... The sound effects are a way of putting diegetic (hope that can be used for more than film) sounds without having to state it in tbe story. They can be used to set a tone (sharp, slanted lines for harsh clicks, ya-da) and, in Japanese, letters are seen as more of a decoration or art form (say Shoudo/Calligraphy), just one of those hard to ideas hard to translate culturally... But for this work there are some variations in use, the shakes on page 3 for example, but feel free to try experimenting some more with directions and placements, all of the ba-dump's are the same size, orientation, and font, but could redily start small and build with the story...
Now mind you I'm not really an artist so do take what I'm saying with more than a little salt. But if you were going for the translated doujinshi then you got it perfectly.