I try not to pin down exact voices for my characters, and my reason is largely that since I'm an admin member of Voxus, a voice acting group that dubs comics (best known for Let's Read Homestuck), we have played around with the idea of dubbing Errant at some point. I'd probably be on the casting bench, and when I'm casting, I try not to take in too many pre-conceived ideas about what characters might sound like and close myself off to interesting and unique interpretations that aren't what I thought, but that work.
In my head, all the characters have English accents, but like... well, they live in an alternate universe where the anglo-saxons never really got a strong foothold, so the language they speak actually isn't English, because there's no England, it'd probably sound more like a cross between Welsh and French... so I guess any accents would be fine, though I think some people might find it jarring for these characters who live in "Brittania" and walk around a fictional city that looks a lot like Manchester to have American accents... given that with online voice acting, you're always limited in how many real British people you can cast, and a lot of people doing fake British accents sound bloody terrible, so I'd rather have good performances and VO of roughly appropriate ethnicity in american accents than the alternative.
Rekki: Has a relatively medium high, but tomboyish sort of voice, probably a bit of a husky sound and generally tends to sound high energy, tense or stressed. I could imagine somebody with a similar kind of range to Ashley Johnson in this role.
Sarin I always imagine with a smooth, level sort of voice, a similar kind of tone to somebody like Deedee Magno Hall or Tara Platt, like the kind of voice that sounds gently reliable and articulate. East asian VO would definitely be preferred, and particularly a VO of Japanese origin with some fluency in the language, which is where casting would get a bit tricky if I went the "British accents" route; British Japanese people aren't nearly as numerous as American Japanese.
Urien has practically been cast. Ryan has already submitted demos because he really likes Urien and he sounds great in the role. Ryan usually plays either slimy scumbags or handsome protagonists (if you play a lot of BL dating sims, you may have heard him in games), so he's kind of perfect as a scummy but handsome and charismatic guy like Urien.
Subo probably has a warm and friendly sort of voice with a laid back sound, kind of a "modern game protagonist" voice type you could imagine being played by somebody in a similar range to Troy Baker or Matt Mercer. I'd obviously prefer a south asian VO if possible, since he's a Punjabi character.