Can I tell my Bishop I consider writing as my “true calling” and therefore should not have to serve in the ward?Actually, that has happened before. I know of two cases--both authors wrote about increasing spirituality, recovery, parenting, and other gospel topics. Their bishops felt their books were of such help to the members of the Church that they wanted them to write more, without being distracted by other service within the ward. (Other than Visiting and Home Teaching. I don't think you can ever get out of that. Nor would we want to, right?)
But I think I'd wait for your bishop to be inspired and extend that calling to you himself, rather than asking him for it.