It looks to me like a shorter alternative route to using the left port gate and turning left at the first roundabout may be possible. Stay in the cycle lane out of the port to where it ends at a level crossing, then turn right onto a short gravel track across the level crossing and left onto the roads among the port buildings. The ones I can see in photos have 30 and 50kph limits (roughly 20 and 30mph) and I would expect fewer lorries leaving in a bunch than from the ferry.
Turn left at the end of the first road, follow the next one to a roundabout and then straight over that and the next roundabout to reach the one where you can turn left onto D940 through Loon-Plage. It's only a fifth of a mile longer than staying on the direct road towards the autoroutes. https://cycle.travel/map/journey/134693
Let me know if anyone tries this, please.
We checked this option out.
It's doable, but the only exit is via the railway line where you could walk your bike over to the road.
In the meantime you will have every HGV from the ship thundering past you
I'd go with the map above version which has been fine tuned by a number of riders