Congrats on making a dub!
I keep threatening to make "Let's Read Errant" but ah man... making comic dubs is so much work... I've been working on comic dubs with Voxus like Let's Read Homestuck, Let's Read Paranatural, Ruby Quest etc. for like 10 years at this point.
One advantage I'd jump on if I was making Let's Read Errant is that I'd have the original photoshop files for all my pages, so I'd be able to make the speech bubbles and sound effects appear. This would be great, because usually when we dub Paranatural, we have to just leave the bubble there and white out all the text and have the text appear.
Streamlining the VAs is never going to be easy. This is why there's a joke in Let's Read Homestuck that the part of Nepeta is cursed. We've had I think four Nepetas at this point (plus one incidental line where we were between Nepetas and Adox, the voice of Jade, voiced Nepeta and nobody noticed). But it does get easier as you build up a network of voice acting friends, who often know other VAs you can call in for bit-parts and cameos. Plus sometimes you'll get somebody who just has an amazing Chameleon voice and can play multiple parts in the same production, which is often really handy. Overall, for managing VAs, the best thing is to make it as easy as possible for them to find their lines and get them submitted. We use a shared dropbox for it.
The main thing I'm fretting over for Errant is sourcing music, really, since it's not a fan production, so it would make sense to use Royalty Free music rather than the freedom we have with Homestuck to dive into all the various official and fan albums for that comic. It'll likely add an extra complication. That and sourcing enough good English accent VAs.... Still... it's tempting to make one though...