Do you know one of the addresses in the keychain? Ledger uses BIP44 derivation so you could write a loop to check different seed phrases and check the resulting addresses against an address you know that should be in the keychain. This way you can type in your recovery words without having to be online.
For example if it were me I'd probably write ...
The following produces the wrong private key! (but is probably a step in the right direction)
In c-lightning the per commitment point / secret is derived using per_commit_point / per_commit_secret from:
The node id / public key (can be found with listfunds, until the channel is purged some time after closing)
The channel database id (can be found with cat ~...
Bitcoin Core 0.18.0 is compatible with any wallet version from 2017, if you want to check you can download any Bitcoin Core release. Bust most probably you are misspelling your password.
You could try also from the console. Use this command to decrypt it for 60 seconds:
In Bitcoin-Qt click Window -> Console.
walletpassphrase your-wallet-passphrase 60