I definitely wouldnt give up on this if there are 600 coins in it. 20% fee to me sounds like
a bargain, at the moment you have nothing!
I dont know about encryption but I came across this:
https://www.quora.com/How-do-I-crack-a-7z-file-passwordAlthough 7zip can uses 256 bit AES encryption which itself can't be cracked by brutal force in anyone's lifetime, but the password that it uses to protected the encryption key is the weak link, simply because it need to remembered by human being.
So all you need is to recover the password, obviously you cannot remember or have an
idea of the passwords you used back then, I would try that recommendation above with
the hope to get 80% of what is in it !