How exactly do you rise through the military ranks? Is it based on the number of military sorties flown, on the number of vonari kills or something else entirely?
Number of military sorties flown. Your military combat service record (how many
successful missions you complete) determines your rank.
Also, since the NDA seems lifted...just how good are those military craft? What's the point in owning one?
Can depend on how you define 'how good'. Their primary design focus is combat. So if your gameplay preference is combat, they will likely be good ship options for you. For other objectives like trade, exploration, mining, crew management (they don't have crews)/passenger transport, energy processing, item construction, plant harvesting, and other non-combat activites, they are either less effective or don't have features necessary to complete the objective(s). They are very fast and agile. They do have design limitations though, such as very limited design options, no crew options, and no cargo bays. But if your mission calls for escorting a capital ship, hunting down Vonari/hostiles, spying, racing, or multiple waypoint patrols, you'll find these ships offer some unique benefits that
some civilian ships struggle to compete with (some can compete if configured/designed for combat focus).