Finally solved.
I must admit I've always read that "default option" as "use the default options of the decoder" and there are a few of them out there with default keys already typed in.
Consider that in my search of the right keys I went to the extent to really "ask the man" and I found out the ones he decoded back in 1918.
Anyway... thanks for the puzzle ;) I banged my head on it for a good while.

Solved! I have all the games in my libray but thanks for the puzzle!

Finally here

Took me few days to figure out that I need to remove formatting
Due to the way the cipher works many tools break the code into 5 letter blocks, same split was used in the puzzle, so I used that as the cyphertext. Unfortunately such formatting breaks boxentriq

The actual keys are
DEFAULTBCGHIJKMNOPQRSVWXYZ0123456789
OPTION
and they work in https://cryptii.com/pipes/adfgvx-cipher which also doesn't break on formatting

Implementation on boxentriq seems oversimplified (hidden conversion of alphabet[visible in grid view] and lack of support for whitespace characters)

I have a question though, shouldn't the transposition key be OPTIN instead of option, since from what I read the practice was to avoid duplicate letters. This is what had me stumped for awhile till I found boxentriq.

I was stuck on the keys for days, I had even tried Default Option as the key but didn't think to try Default for the alphabet key and Option for the permutation. The nested zip trick with concatenated images was clever.

Thank you!

couldn't get past step 2.
Did change the extension to .zip, but it gives error/damaged message, and could not do anything.
How are you supposed to open these?