Candice, you have two ideal candidates to successfully meet your SV meal plan timing requirement.
Flank and skirt steaks are usually about the same thickness and will require at least 8 hours of SV cooking. For very tender results, i’d cook lomger, up to 12 hours, or even longer for additional tenderness. In my experience tenderness is a personally subjective qualification and should be based on your experience to decide.
What’s thin? Thin steaks might be cut from baby beef and be about half the usual thickness from a mature animal, although i don’t often see thin cuts other than in spring when dairy farmers are culling their herds.
Cooking temperature depends on your undisclosed degree of desired doneness. I cook them to medium-rare at 131ᴼF/55ᴼC. For food safety i would not cook meat for that length of time at a lower temperature.
Cooking from frozen is always an option, but not necessarily better as it extends the cooking time required for a typical flank steak by about 2 hours to achieve a similar outcome.
Do well, and stay safe.