I know I said I wasn't going to play it again, but I magically managed to 1cc EoSD! I even still had 3 lives left. Now, I decided I wanted to try EoSD hard and compare it with normal mode and ended up 1ccing it on normal. Oh, here's the comparison:
Stage 1: No difference in the beginning, but the youkai that actually shoot have two rows of bullets now. Midboss Ruima seems mostly the same aside from the new spellcard.
Ruima: Some things have more spread, some attacks shoot more rows. Darkness Sign is noticably harder.
Stage 2: Mostly just more bullets, but Daiyousei's attack is now acting simaliar to one of Remi's noncards, only much easier.
Cirno:First noncard has some extra circular spread, and the first card changed completely. Perfect Freeze seems to have more bullets as does Diamond Blizzard.
Stage 3: Some enemies now have extra circular spread, Midboss China's card took a leap in difficulty. She shoots more bullets in her noncard, too.
China: many spellcards changed, and a new one is added in, not much aside.
Stage 4: just more bullets flying everywhere. Book lasers appear more abruptly.
Patchy: Non-directional laser sped up. Added spellcard. Fire sign has the fire hitting you from both sides at the same time.
Stage 5:almost no change except Sakuya's spellcard.
Sakuya: difficulty is really high. maid secret skill is much harder than before.
Stage 6: Circular bullet spreads now range in speeds. Not much else
Couldn't write about Remi due to insufficient info. Note I'm just including the most noticable changes
Next up: clear EoSD Extra then 1cc PCB.