I’m considering going back to school for my DNP. I’m very interested in working in dermatology. I don’t see that as an available specialty path. Does anyone have any advice on how to acquire that knowledge, and what DNP specialty to choose instead?

Any nurse who is looking to work or train in dermatology should first, join the Dermatology Nurses' Association-https://www.dnanurse.org/. There you will find educational resources in all areas of dermatology. There is also a peer reviewed journal that you receive as a member.
Besides the education, there is a great chance to network, especially if you attend the annual meeting. It will be in Orlando in early March 2025.
Nurse Practitioners also have their own group/committee and there are advanced educational resources presented at the Nurse Practitioner Forum.
Many dermatology practices are starting to have Dermatology Nurse Practitioner training programs. The first one was established in Massachusetts at Beth Israel Lahey Health.
Best of luck!!