Heh, I think I've tried every "Cipher Solver" that I could find (dcode is a good resource for testing) but all that comes out is gibberish.
I might be doing it wrong though (the upper cases and lower cases are grouped as the same letter in the online versions, as well as the numbers seem to stick around).
So when I do it, it doesn't matter if it is "A" or "a", it is registered as a the letter "a" regardless, and numbers aren't always processed either, so a "6" will stay a "6" at the place it is. If it is the Ceasar cipher, then we have to manually decrypt it, which is beyond my ability (also I'm too lazy, automation is so much more simple ). But there are codes without numbers, and I believe codes with unique letters (no repeats) and they still just turn out gibberish in the online decryption software I use.
One would think that at least one code would make some sense or make out to be some sort of understandable words or phrases using any sort of Cipher solver, but alas, nope. Nothing. Although, your reasoning sound solid.
EDIT: For instance, here's one:
XQrdGv
And it gives this result at this site, I do a "Brute Force":
https://www.dcode.fr/caesar-cipher
"Brute-Force : all shifts are tested, text is limited to the first 250 characters. To keep punctuation and space, please indicate the correct shift found (+XX)."
+15 IBCORG
+3 UNOADS
+2 VOPBET
+16 HABNQF
+17 GZAMPE
+24 ZSTFIX
+25 YRSEHW
+12 LEFRUJ
+21 CVWILA
+13 KDEQTI
+6 RKLXAP
+20 DWXJMB
+22 BUVHKZ
+23 ATUGJY
+4 TMNZCR
+5 SLMYBQ
+19 EXYKNC
+18 FYZLOD
+1 WPQCFU
+10 NGHTWL
+14 JCDPSH
+9 OHIUXM
+7 QJKWZO
+8 PIJVYN
+11 MFGSVK
Gibberish.
Decrypting doesn't make any sense when the text is still encoded. If the text is encoded it's either Base62 (0-9,A-Z,a-z) or Base64 (+\=).
I just don't think it's a coincidence Josh is holding the cup right into the camera after putting down that book.
Also, to my knowledge, it's the first time they officially acknowledge the codes and want to give a hint. Release isn't that far and i doubt they will continue embedding codes afterwards.