After basically wasting the afternoons of a week, I didn't manage to get the tool work. It may matter that I've tried to build a safe Linux on an USB stick for the job and that ate up a lot of time too (and the result was far from perfect), but this solution to obtain the recovery phrase is not for me.
Now I know that I better don't mix Monero with Ledger. Thank you very much for opening my eyes, I may have made a mistake otherwise.