Downloaded the zip twice, checksums match, archive can't be opened and PeaZip asks me if I would like to try decrypting the archive.

Did you tried with winrar?
It could also be a problem with your browser while downloading it. Try again or try to use winrar, if it doesn't work, we aways recommend to get the game through itch's app because it's more reliable than the browser, since downloading all at once it's over 4gb big.