Good job, Schonbergian! Unfortunately, when you deal a lot with choral music you can't help coming across the very worst of 'contemporary engraving' every day, even in professional publications. This is a very pleasing counterexample!
Because you asked: I've got a few thoughts.
1. Vertical spacing: The Breitkopf edition is beautiful. As you suggest, I would move the lyrics closer to the staves. In the first system, Cantus, I would even move the word 'Domine' closer to the stave, thus 'breaking' the lyrics line, but avoiding that 'Do-' floating in mid-air. (The extensive use of C-clefs in old editions of choral music resulted in less notes outside the staves so the vertical position of lyrics was then not so much of a problem.) Concerning the spacing of the staves: I think this is a matter of balance. In the Breitkopf edition, the space between the systems is much smaller, white space on the page is more evenly 'spread out'. In your case, I would definitely increase stave spacing.
2. Type area: I would increase all, especially left and right page margins to achieve more harmonious proportions of the type area – and to shorten the systems, which are a little too long for my taste. If that results in too dense horizontal spacing again, why not go back to the smaller system size you started with? Choir singers, at least in Germany, are used to read music from quite small staves used in piano reductions. On the first page, I would add even more space between title and music. (I found this helpful, when I started engraving:
https://en.wikipedia.org/wiki/Canons_of ... nstruction)
3. Note values: I feel more relaxed when I read the old 'double' note values used in the Breitkopf edition. People may have to get used to it again, but is that so much of a challenge? There are so many beams and black noteheads in your version... I always feel like those many white noteheads in old editions of renaissance music give me more space to breathe! Also, less black and more white noteheads and less beams might make the music appear less dense. Look at the quarter notes in the Breitkopf edition: they are more densely spaced then your eighth notes, but still don't seem too close.
3. Fonts: This is a matter of taste, of course. I like those fell types but I think they don't combine too well with Cadenza (which I love for it's clarity and simplicity). Perhaps a rougher music font like MTF-Haydn would be a better choice? Or Cadenza plus something like Century Schoolbook, or even Minion Caption? The numbers of the fell types are a little harder to read, I would increase the distance between measure numbers and staves.
Voice names are too large for my taste.
4. For my eyes the brackets are still too far from the staves. There's too much space between the start of the first system and the 'vocal range indications' (don't know the english term), but not enough between these and the clefs. Perhaps use smaller noteheads for them?
Anyway, the score as it is now is already much better then many! Go on with your good work!
EDIT: I just saw Fred's post, sorry for any duplicates. I agree about those points after anything. And no, I also don't think lyric extenders are optional.