I have started my second playthrough and only now it feels easy (deathmarch again), because I knew exactly what to do and how to proceed. The first playthrough was really unforgiving though and I liked it. The "new game+" mode is on the way. Maybe that might turn things around for you.
Also, you are right about Skyrim. But Skyrim's difficulty is/was fucked up all along. You really don't need to be on your second playthrough to not feel challenged anymore. Looks like they wanted to compensate for the bullshitty enemy scaling of Oblivion but overdid it. Witcher 3 really doesn't fail like that. I felt that Gothic difficulty in this game and it was awesome.
Anyway, one's sense of enjoyment within a specific genre or in the case of any game gets blunted over the course of time. I understand where you are coming from regarding that.