Honestly, pretty disappointed here... Spoilers ahead.
Prushka's character is really not that different from Malaluk or Mitty... in fact she's kind of a combination of them, which immediately made me not care for her all that much. Her design is fun and very charming, but for such a central character, she plays a role we've already seen before. Bonedrewd, on the other hand, is a fascinating character and it's a shame we don't get to see more of his past: what led to his fascination with the abyss? Does he know the other White Whistles? What made him throw away his humanity in pursuit of his research? So much good material that's untouched upon.
So many strange story points too... Riko and Reg swore revenge on Bonedrewd. They hated his guts! They fight him and even end up killing him. Twice! (apparently forgetting that his white whistle is the only thing that can get them to the next layer?) Then they forgive him at the end?! Nanachi too which really surprised me and made me question his character. And the way they leave Bonedrewd and the Minature Garden behind seems to unfinished and rushed. Riko cares so deeply about Prushka after spending a few hours (days? the passage of time is real wonky here) with her, but never recalls Malaluk whom she became great friends with before...
I know I'm really eating at the plot here, but I felt so confused by these characters and their decisions. Maybe I need to watch it again, or maybe the Manga has additional details that movie cut out.
Regardless, the settings, creatures, music, and character design are superb. The team did a phenomenal job with the minimalistic 5th Layer and achieving its desolate atmosphere. Bonedrewd is a compelling character and villain and seeing his plans unfold was a joy to watch. Definitely a great and memorable chapter in the series, but it feels oddly paced, rushed through development, and lacking the tight storytelling and originality that made the anime series so compelling.