Steam Bullet Fest, from now until June 15
8d 22h ago by lemmus.org/u/sundray in shmups@lemmus.org from store.steampowered.com
Note: This event is a mishmash of shmup, survivor-like and bullet heaven games. Check in with your favorite Steam curators to filter out the chaff.
Also, there is a "shmup exclusive" wishlist on the price tracking site IsThereAnyDeal: https://isthereanydeal.com/uid/48zn4/waitlist/which might help you find what you're looking for.
Definitely use those filters. There are so many rogueli(t|k)es mixed in as well. Lucky for my bank account I don't game on my crappy laptop.