this channel's started posting interviews with indie STG devs on their workflow, design processes, etc, starting with a couple of the must-plays of the last several years—for a genre as codified as this one, you'd expect there to be more first-hand design discussion out there to reference, but for whatever reason (legacy print outlets being limited on what they could print; reticence on the part of devs/publishers to talk shop, or talk at all; general lack of opportunity, etc) there aren't a lot of nitty-gritty details out there for even the most celebrated games, so conversations like these are welcome:
These interviews are great! Boghog's Shmup Workshop video series and its corresponding document are fantastic too, if you want more STG design tips from a designer's point of view: