So, say someone with abilities close to the old Ichinose-type classification we used once upon a time, but capable of Bankai? I assume that would be high-end Aristoi level?

I'd personally interpret Aristoi as average Captain-class, and Archons as Advanced Captain-class, seeing as we wanted to make the cast more reasonably strong.

I'd like to propose Hikaru Maebure for the Coven's Anarch, as he's been made (and revised) with such a role in mind.

Well, most Aristoi are Kidou Specialists and many of them can't even use their Shikai, since they've dedicated their lives to the pursuit of magic, so the chances of one of them knowing Bankai is exceptionally slim. Megumi for example doesn't even bother carrying her Zanpakuto on her person anymore for that reason, besides, this little tidbit about them works in their favor by making them further distinct from the Gotei 13.

But yeah, as a whole the Aristoi tend toward lower-end Captain Class, with some being weaker or stronger than others all depending upon what their magical speciality is. Most are probably Ichinose-class, but some, like Megumi are more equivalent to a low-end Captain-level Shinigami such as your very own Hisagi, Kenji.

Neophyte suggestion

Looks great, I actually think this is a better image to represent the coven as a whole than the manga image from Black Clover that we currently have.

Wait where exactly are they getting these faceless neophytes?

And what are their plans (if we're still keeping this idea) for the Spiritually Aware humans (now disappearing, following what Sei said)?

Former rank-and-file Kidō Corps members + fresh recruits from the Rukongai.

The same place as they got these faceless neophytes.

Neophyte R Us is going out of business, so the Coven got them for a great bulk discount