I've been thinking that maybe it doesn't matter how well received the comics are, Yen Press is most likely going to finish re-printing all the issues. (I'm not sure about the special issues, though.) Since Yen Press is also a manga distributor for English-speakers, that means they tend to finish translating and (re)printing all volumes of a manga/comic series.
And hey, check this out! I use Goodread dot com to keep track of my reading, and I saw they already put up the synopsis for part 5, vol 1 and 2 before Amazons:
Part V The Book of Elements vol. 1:
An Enemy Redeemed?
The girls are growing into their powers, but possibly not quickly enough...? As their elemental magic swells, the girls begin losing control of their frighteningly increased abilities. Possibly even more frightening, though, is that Cedric is back...and getting a second chance from the Oracle?!?
How will the girls cope when one of their oldest and most dangerous foes sets up shop in Heatherfield...?
Part V The Book of Elements vol 2
Running Heatherfield's Ye Olde Book Shop isn't the new life Cedric wanted for himself, but it may have given the perfect opportunity for revenge! With the Book of Elements and the secrets it contains, Cedric's found the power he needs to finish the Guardians once and for all, but is that what he really wants...?
It looks like they haven't gotten vol 3 up yet, but it says that the expected date of release so far is January 22, 2019
No, wait...they just put it on Amazon and it's also 2 vol. I don't know if they're condensing it down to 2 omnibus, or if they haven't gotten to vol 3 yet.