I feel the whole "true angura" thing (I mean acts like Strawberry Song Orchestra, Guruguru Eigakan, Inugami Circus Dan, Dagashi Kashi, Ningen Isu)'s mostly about bands drawing their influence from retro aesthetics from the period between like... the '30s - '80s or something, possibly some degree of nationalism, and also works of authors/artists such as Edogawa Ranpo, Shuji Terayama ( + other stuff connected to that whole Tenjo Sajiki gang), Suehiro Maruo etc. + I guess in some cases acts connected to the whole 80s new wave scene, especially the Nagomu Records label, also had some influence... Groups like Zin-Say, Karate Bakabon or Uchoten for example seem to have had some impact on the bleep-bloop side of things and I guess they could arguably count as some sort of pre-VK predecessor to the angura scene in a sense.. I see some influences from those groups in acts such as Muchi Muchi Anago, Shinjuku Gewalt, Metronome etc.... And of course a part of it is also just kind of arbitrarily determined by means of the bands certain bands mostly play with, I guess. Probably something like that whole funky white raccoon face aesthetic's a better indicator of "trve" angura-ness than kimonos, although I guess angura bands do like their uniforms from time to time.. Still, I'd definitely not consider acts like R-Shitei, Memento Mori, Orochi, Kagrra, or Kiryu to belong to the sublabel. R-Shitei's mostly just flirting with the retro aesthetics from time to time but doesn't really reflect any of it in their music or lyrics from what I've seen and heard..
The whole "darker and more formal" thing's sort of been memed into existence by people who somehow consider acts like Deathgaze to be angura or eroguro, and it's not really accurate, because some prominent and essential angura acts like Guruguru Eigakan or their offshoot Taiheiyou Belt, and even acts like Shinjuku Gewalt aren't actually all that dark or formal when it comes to the music. If anything, ''nostalgic'' would be a much better descriptor for the genre as a whole..
Im 1000% sure i could probably write a longer and better post about this but i'm sort of half drunk rn lmao sorry, i hope you caught my drift though.... maybe sb else will swoop in as well and add to this