Thank you for playing! And I respect your opinion ^^
The game strays quite a bit from the usual shooter formula, and lack of scoring mechanics is intentional. As an attempt at "Horror STG", the emphasis is put on aesthetics - atmosphere and tension. So the pace needs to be somewhat slower, or else it would fall flat. Though the player does control the game's pace to an extent - fully destroying waves faster will speed up progression and give you more time to beat the boss, before walls fully close in.
Regarding walls - personally I hug them a lot myself in other shooters. I understand the muscle memory can play some nasty tricks with how the walls are instant-kill. Yet, there needed to be a way to implement the "limited" theme. And I think mechanics like "Limited ammo", "limited fuel", "limited time" are very, very hard to implement in a fun way. Most of the time they'll just hamper the player's abilities and act as an unfun inconvenience than a threat that you'll need to play around and strategize against. The walls in Perdition Sphere mechanically act as a soft time limit. Aesthetically - they're supposed to really mess with your head, gradually induce a sense of dread and panic. You don't have much time left. If you don't shoot down the next boss - the walls will close you in. It's do or die. If the walls only dealt minor punishments, would it really have the same effect?
I think the main problem here is a player's expectations. Your games are bloody fun! They're fast-paced, and there are always mechanics and tools to keep the player entertained, and ways to strategize to reach a higher score. They're fun in their own way. For Perdition Sphere, the goal was to make the player strategize through the challenges imposed on each stage and reach one of the endings - and make sure they feel something from it. This is why the more complicated mechanics such as multipliers and item collection aren't present, as they would detract from that experience, and also scare away people who aren't that used to shoot'em'ups but who could really enjoy the aesthetics.
That said, and regarding enemy tankiness - the game could really use more enemy variety. I wouldn't use popcorns in every wave, but sometimes they'd really make sense - sadly I couldn't find or make proper assets before the deadline. Not sure if I agree on boss tankiness - both stage 1 and 2 bosses take about a minute and a half to beat, which I think is about the same time to beat an average early stage touhou boss..? (If you had trouble with some particular boss or its pattern - please let me know.)
TL;DR - I understand why a player and developer of caravan shooters and other fast-paced games wouldn't enjoy Perdition Sphere's gameplay so much. I do think it's a matter of taste - perhaps in an alternate universe, another rolling crow would have mixed feelings about perdition sphere being too fast :S Thank you for playing and sharing a detailed opinion, hope you don't mind a not-so instant-kill wall of text in response :P