When unlocking Crew the first time after clearing sector 50, Masteries should be your primary focus – Masteries multiply your gained XP significantly, allowing for further pushes and higher total Stat and Rank levels, meaning many more upgrades. Your first mastery will be +3 Starting Biosleeves, followed by the 30x Masteries in the following order: Acumen, Ingenuity, Proficiency, and Engineering.
Why choose +3 Biosleeves instead of 30x XP? It may seem like a trap, but here's the deal: 30x XP translates to roughly 21 extra levels in a main stat or only 10 in a secondary stat. While it's undoubtedly useful, it doesn't hold up to the significance of extra biosleeves.** If you can achieve 31 stat levels with a biosleeve, it's already more valuable than 30x XP**, considering your goal is to maximize mastery levels.
Attaining a mastery on your initial run is quite challenging and probably not feasible. Instead, focus on reaching the highest possible level with the biosleeves you start with. It may take one or two resets before you can achieve a sufficiently high level – You’re aiming for 85 stat levels with a single sleeve before retiring.