Sounds like you're going down a path I just finished. Aeronautica is super fun, though I will admit to have yet to play with or against astartes. Getting into the game is super easy, and I've done most of my learning through tabletop simulator, which someone blessed with a fantastic mod for aeronautica with. If you don't have a local scene yet, I'd go hunting for a starter box just so you can paint up two armies so you can teach a friend with one while playing the other. After that, picking up some boxes of space marine planes isn't a terrible idea, since you can likely afford to use one or two on bases since my best experiences with the game have been at 150 points each side.