The overnight crossing is legal rest
Article 9(1) of Regulation 561/2006 lets a driver who accompanies the vehicle on a ferry take a regular daily rest period on board: the period may be interrupted not more than twice by other activities, not exceeding one hour in total, and the driver must have access to a sleeper cabin, bunk or couchette. On an overnight Baltic crossing that makes the sea leg legal rest rather than dead time — which is exactly why the sea route beats the land route on this corridor. For a regular weekly rest the journey has to be scheduled at eight hours or more with cabin access. A crossing sitting barely above that threshold is not something to build a plan on; one scheduled well clear of it is.