I only used CDG as an example.
As regards the 757, would having 1 getting maintenance not be sufficient. There is 2 757's based in SNN, so if they alternate daily eg 1 does JFK+Shorthaul route and the other does BOS+Maintenance and they switch every day? Also JFK is only 6x weekly (exceptions made in the summer season eg tomorrow) so extra maintenance could be completed on the Tuesday.