We just went through a softgoods (bag) project and none of us had much experience doing it. We found an independent softgoods prototyper (ex-Nike) that we worked intensively with, who helped turn our initial sketches into a real thing and is currently working on a tech pack for us to quote out.
I’d say finding a soft goods prototyper would be useful both in educating yourself and in making a successful bag. You can do paper prototypes yourself as well - this is what the soft goods prototyper will often do when figuring out patterns. Its also useful for gut checking your proportions and ideas.
Thats’s about all I know but others on here will be much more knowledgeable.