Thank you so, so much! (edit: errr I didn't mean this to be such a wall of text whoops)
The theme isn't incorporated into gameplay too much, though I did try to add patterns which would give you very little breathing room, however I'd argue I did incorporate it into the story at least to an acceptable degree. When I pitched the idea to the composer and the character artist I described it as "Introvert Vampire Sleeps in his Arcane Reinforced Coffin Until Space Age - Doesn't Wanna Leave When a Giant Space Fleet Attacks", the original idea was that the main character confined himself into the coffin of his own accord, quite simply because he really doesn't wanna deal with people :)
The health system I think was subconsciously inspired by a really crazy shmup called Akashicverse. In this game you have a shield that can be toggled at will which has % energy, recovering over time. If you get hit without a shield you lose on the spot and there are no lives, though when it's off the energy also gradually recharges, so one way to play is to turn it off for slower attacks that can be easily avoided, and keep it on for wide or otherwise dangerous attacks and try and evade them with lesser risk of instant death. Coupled with its unique system of special moves which requires fighting-game style inputs for powerful offensive/defensive attacks, it gives you some leeway and doesn't demand perfect dodging for the amount of fast bullet hell you have to deal with, so long as you adapt and try to understand what you're doing. Actually, considering the points you loved about our game, I think Akashicverse might be a very interesting game for you to take a look at so let me link its steam page: https://store.steampowered.com/app/1225610/kashicverse_Malicious_Wake/
I'm happy you enjoyed the pacing of the attack patterns! While designing them I aimed at making them challenging to make the best use of the hp system, but also dodgeable, and from my tests about 90% of the patterns can be dodged without getting hit (while the other 10% I think are possible, but extremely hard so I couldn't do it myself). I wasn't sure if the difficulty level was appropriate or not and while testing it felt too easy with the HP system at times, while testers had varying opinions on it, but most people seem to have enjoyed it so I'm glad!
The engine we used, Shmup Creator, currently doesn't have as many features as I'd like for customization, for example it's not possible to change the cancelling duration of the vampire mode and the way healing works uses a workaround that isn't easy to control, but the developer is open to suggestions and was very helpful during development, so I'm sure it will be possible to tweak more stuff in the future, I definitely want to experiment with the concept more.
Thank you for your detailed comment and kind words! It honestly made my day :)