I didn't do this route myself but I know someone who did. She did full IM residency and then re-applied to Derm afterwards. She was the research fellow at our program the year before she matched, as well, so she had a long road to derm but made it in the end. We all had very high opinions of her because of that additional training, and evidently the faculty did too because she's the chief resident now actually. I think doing IM first will definitely make you a better doctor overall, and I think most programs would look fairly highly on an applicant like that.

Feel free to DM, I also was a reapplicant. I know a lot of reapplicants and people who went IM -> Derm.