General:V./Solution

The fifth doctrine of The Soft Doctrines of Magnus Invisible has a cipher in plain sight, the huge chunks of text that are seemingly random letters.

I will give the solution to the first chunk of ciphertext, and then post the three full solved ciphers.

To solve this cipher, you must examine the Doctrine:

AU-IM REFLECTS!

This is an easy clue to the easy cipher. What it suggests is simply to reverse (reflect) the letters of the ciphered text. So -

GTSMGQPVKJTHMCZJAKVGMJKSUFZQTH
GQJKHSJMHRMTGTHMCZJAKVGMHJQLFQGQTH
GQIUPRPUHQIJMZJAKVGMGAJSUI
GHKJOHQFWQGJMZJAKVGMJFMJ

becomes -

HTQZFUSKJMGVKAJZCMHTJKVPQGMSTG
HTQGQFLQJHMGVKAJZCMHTGTMRHMJSHKJQG
IUSJAGMGVKAJZMJIQHUPRPUIQG
JMFJMGVKAJZMJGQWFQHOJKHG

Next hint is -

E_____ 432

- which, according to Douglas, is a difficult clue to a lore thing and a real life thing and a song reference. I'm not aware what exact lore thing or song reference it is, but the next step for the solution is to substitute the letters using using the alphabet split into 4 letter blocks and reversed (UVWZQRSTMNOPIJKLEFGHABCD), without X and Y, as is tradition. The double I's in the text therefore mean Y's.

THEDRAGONISBOUNDWITHNOBLESIGHS
THESERPENTISBOUNDWITHSHIFTINGTONES
MAGNUSISBOUNDINMETALFLAMES
NIRNISBOUNDINSECRETKNOTS

Which are the lines that all the new Doctrines ended with, and the full old Doctrines text -

THE DRAGON IS BOUND WITH NOBLE SIGHS
THE SERPENT IS BOUND WITH SHIFTING TONES
MAGNUS IS BOUND IN METAL FLAMES
NIRN IS BOUND IN SECRET KNOTS

Using this solution, the second ciphertext, which is the biggest, gives us (punctuation added by me):

DO NOT LOOK TO THE MOONS
OR THE PALEST REFLECTIONS,
SMALLER, COLDER, DEADER.
DO NOT LOOK TO THE TOWER
OR THE DANCERS ON ITS ROOF
WHO LACK RHYTHM.
DO NOT LOOK TO THE WHEEL
OR THE SAILBOATS ON ITS VEIL
SANS NAVIGATORS.
DO NOT LOOK TO THE GREY
OR BE FROZEN BY POTENTIAL,
LOST IN MUSIC.
DO NOT ADORE THE JAILER
OR ABHOR THE JAIL.
ONLY BY SUCH PRUNING
MAY WE GROW TALL.
DO NOT LOOK TO THE PLENUM,
WHETHER IN ASSEMBLY OR IN EMPTINESS,
OR BE MAD WITH COLOR.
LOOK TO THE INVISIBLE
AND THE MISSING FOUNDATION
AND THE MYTHOPOEIC GAP.
RETURNING LOVE, HOWEVER FLAWED,
IS THE FIRST TREMBLING STEP
OF THE PRISON BREAK.

And the final block of text deciphers as:

LIFE REJOICES IN LIFE
LIGHT REJOICES IN LIGHT
REJECT FIRE AND BLOOD
REJECT MER AND MONKEY
REJOICE IN TRUTH
REFLECT LIGHT
EARN FREEDOM
YEARN IMAGE