Silvance, as a government/military linguist you should have a JLU (jlu.wbtrain.com) account, or be able to get one. JLU has a number of self-paced Dari Language Survival Kit courses precisely for people in your situation, and they should be a good start. That site might have other Dari resources as well. You might be able to get access to the Rosetta Stone Farsi course, which is less directly relevant to you but pretty good, either through JLU or through a public library system - many of them seem to offer Rosetta Stone to library subscribers. That would be another good start. Among the lists already given for Farsi, the Easy Persian, University of Texas, FSI, and DLI resources are good.
See also the International Assistance Mission's Language and Culture Program site at
https://www.iam-afghanistan.org/lcp/online-resources/. It has a learner's grammar for Dari, a Dari-English dictionary, flashcard sets, etc.
There should be no difference in ease of learning for you between Farsi and Dari, other than the availability of resources. The two are more or less mutually intelligible, and the written language should be very close. The differences, I think, are mostly minor ones - some pronunciation differences, and some differences in common vocabulary, the extent of which might be comparable to differences between U.S. and British English (the "truck" vs. "lorry" kind of thing; compare to Dari motar vs. Farsi khodro for car). Otherwise, any Farsi you learn should be directly transferable to Dari, so you shouldn't be reluctant to make use of Farsi resources, and adjust to Dari vocabulary differences etc. as you go. I'm not really sure whether the proportion of Arabic loanwords differs between the two - Farsi is about 40 percent Arabic, as you probably already know. In any case, those loanwords make it easier for someone who already knows Pashto.
Movies and TV might help with pronunciation. Do you do Netflix? Try "A Girl Walks Home Alone at Night" (great vampire movie, if you're not burned out on vampires, and the dialogue is not too fast). Netflix has a lot of Persian-language movies. (I would avoid "Taste of Cherry" if you're easily depressed, though.) YouTube, of course, has a lot of Farsi movies as well, and I think there's an Afghan movie channel, though I'm not sure how much of it is Dari vs. Pashto.