I know you said "keep it all French", but go with a Suntour.
I used to save all the cheap Simplex derailluers that had broken jockey wheels - until I realized that it was damn near every one I saw. They are junk, and are either broken or waiting to break.
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"Love is not the dying moan of a distant violin, it’s the triumphant twang of a bedspring."
S. J. Perelman