A few speculations on the back of recent discoveries:

– There’s more content in z340 that we haven’t found yet. The same might be true for z408.
– If we apply the z340 key (thanks to Oranchak et al.!) and allow symbol mirroring/inversion, we can find interesting strings in z340.
– I suspect the second half of the cipher and that out-of-place "LIFE IS" might need to be rearranged and handled differently somehow.

About z13:

– It’s highly symmetrical; hints at the use of mirrored writing.
– It looks like a regular expression.
– The circled 8s don’t appear in any other ciphers so they are probably meta characters and not substituted plaintext characters. I interpret them as meaning 8 columns/characters.
– The "AENM" and "ENAM" make up two 4-column blocks and hint at the use of anagramming..
– If we add up the numbers and word lengths in z13, we get 34 (do we need to rearrange the cipher in a 10×34 grid and section it using z13 as a kind of column header?).

This is all a bit speculative but maybe we can apply the "z13 regex" to z340 by
1) placing a 34-character window at the positions in z340 where a "K" and an "M" are 8 symbols apart,
2) mirroring/inverting the symbols in the matching section (there are two matches. I arbitrarily ignore the first one for now),
3) substituting the inverted symbols using the z340 key

Here’s the output of a quick and dirty POC:

$ python z13_poc_1.py
MATCHING CIPHER LINE
>VUZ5-+|c.3zBK(Op^.fMqG2RcT+L16C<+
 
MATCHING CIPHER LINE MIRRORED
<VUZ5-+|C.4zbk(OP^.FMQG2rCT+l16c>+
 
MATCHING CIPHER LINE MIRRORED DECIPHERED
I O S R V S H E Y N E A E I T A I O N F O Q T M r Y R H A R D E N H

This still needs more work but notice that the plaintext ends with "T M r Y R H A R D E N H". That’s quite close to both "TRY HARDER" and "Mr HARDEN", isn’t it?

EDIT:
– Added pastebin link to python script