I've been conversing with someone via PM about an IM to derm pathway. Perhaps you are the same person under a new username. Or perhaps you are a different person.
But my advice to both of you would be the same.
You cannot go into this doing the bare minimum and expect to match dermatology with success. The question is why would a dermatology program take an IM resident when there are so many qualified 4th year medical students they could choose from? (This is particularly relevant if you've failed to match into dermatology as a medical student and then opted to do IM first with an eye to reapply into dermatology) Selecting a highly qualified 4th year medical student means there are no funding issues that the dermatology residency program has to worry about.
Your Step 3 score and IM board scores will not factor much because you are being compared against an applicant pool that doesn't have those scores.
As phillymed777 mentioned, the typical pathway is exemplary research. Most people need to take additional time off AFTER IM training to do this research. This can sometimes be years of research. During this time, it will give you an opportunity to potentially do more away rotations. Beyond the research, this is an opportunity for a dermatology mentor and a dermatology program to get to know you very well. In this scenario, you become a known quantity. At the rank list meeting, your dermatology mentor can go to bat for you and say "Yes, there are lots of very qualified MS4s out there applying for this coveted spot but I've worked hand in hand with nalasimba for 1 year/2 years/3 years/etc etc and I know what he/she can bring to the table." That is the true advantage of research and finding a derm mentor.
But the point is, you cannot expect to complete 3 years of IM residency and then waltz into dermatology. There will be a significant amount of work and networking involved. Is it worth it to do this when you can already start practicing in internal medicine or pursue a fellowship off the tree of internal medicine? That is the more persional question that only you can answer.