Dorthe with the (umlaut?) above the o is fun to look at, and say ("DOOR-tuh"?), but I think English speakers would automatically try to say this name to rhyme with "north". That's still pretty fun, but I don't think a lot of people would find it an attractive sound.
What about
Dorthy as a compromise? It's got the brief quirk of
Dorthe and clarity of pronunciation like
Dorothy. Although admittedly it does make me think of big teeth, I still find something about
Dorthy really appealing.