All this sounds great to me! I think you've got a really rock-solid plan, I think your scope might be a little wide but it also sounds easily trimmed, and you clearly have a real passion for the schmup genre.
If I were to give two suggestions, I'd say:
- If you have the time, I'd love to see some accessibility options (stuff like being able to put auto-fire on a toggle, decrease game speed, replace detailed sprites with flat colors for readability, etc.)
Nothing in your document really jumps out at me as "THIS is what sets this schmup apart", it mostly just sounds like a further refinement of the formula. I concede that I'm not at all a schmup person, so maybe to someone whose more familiar with the schmup formula there's something that's clearly really different in there (I'd guess that bomb gauge?), but it didn't come through to me. Though, again, I really don't know what I'm talking about when it comes to schmups.