Stefan has been speaking with the door company, who suggested we check their website to see if there is another door type that we might want instead of the vertical planks we had been contemplating.
One possibility is a much more traditional Swedish door, as shown in this post. The one thing I didn't like was the 45º angle; the house is pretty much all right angles with only the roof slope as relief. Adding this kind of door might be a nice touch or it might be jarring.
But we have a better solution in the offing. Stefan sent me an email:
"The team at Designa Doors suggest to try a type which is the same look as the door you have, but with wood for the front coverpanel with higher initial humidity already in production. Until now they have used dried wood panels which have low humidity, which probably is a main reason for the big increase in the width-size of front panel. On the new doors they produce for us now, they switched to wood with 12%-14% humidity instead, and also treat them with very much oil at the factory when they produce them. With this solution the doorpanel should have less increase when it become wet. They will be more prepared for the "outdoor-climate" at your house."
I think this is the best solution. We get to have the original lines of the house, but the doors will be a lot more able to withstand exposure to island weather. I also have to say that Stefan and Ulf have been very responsive and helpful!