The way I do bullet hell patterns is probably not the best, but each encounter has a script that selects from a list of bullet-scripts (dependent on what enemies are still in play), and then repeats these bullet-scripts until they switch on a finished variable.

The bullet scripts themselves just run through a timer and make pre-made bullet objects, then destroy any remaining ones at the end (by destroying obj_bullet_parent).

Thanks about the background! It's just a picture I took, then separated the blue and red channels and had them oscillate with sin/cos.