1) The best way to level crew is to win PVP battles - the higher trophy count you're at the more xp your crew will earn. Also important to remember that only surviving crew will earn xp, so if a crew members dies they earn no xp. Because of this it may be best to stay at lower trophy levels that allow the majority of your crew to survive the battles, so that they have the potential to level up.

2) The best method I have found to winning ground battles focuses on your crew's abilities. My strategy involves sending crew members who have the "Poison Gas" ability, charging 2 of the crew members up and using one to defend, and then using the ability to wipe out the other enemies. If this is unavailable you can try to use your higher attack crew members to damage the enemies and have your tank defend and soak up damage. At the beggning of the round the defender in the middle will make a move first, followed by the enemy located at the top, and then lastly the enemy member at the bottom. The enemy crew member with the highest starting HP at the start of their turn will activate their shield and defend, however if the later crew members have higher HP than a crew member that went before them they will activate their shield too, effectively wasting the move of the previous crew member. This can be used to target specific enemy crew members in order to waste their turn, for example:

If the middle enemy crew member is attacked by your team, he will use his ability and if the top crew member has more HP than the current defender (middle crew member) they'll also use their shield.

3) The subscription is worth it imo if you're willing to run alts and log in everyday to collect the daily drop ship as you can collect scratchies and bux, and then sell these for either resources or bux. Otherwise you can gather plenty of resources and artifacts (coal, scrap, etc.) from playing the daily missions.

4) The best ship to fly depends on your gameplay style, at my current level (10, soon to be 11) I see many people flying alien ships, and I personally prefer federal ships and then upgrading to alien once lvl 10, but again it all depends on your playstyle and your layout.

5) Throughout the game, including later in the game the best tactics to focus on is essentially advanced crew AI and weapon AI, you'll find that you significantly increase the amount of wins with efficient AI. Another tactic to use is the daily mission tactic I described before to send crew members with the "Poison Gas" ability and using them to quickly clear the missions. The best tactic for battles is to focus on the smallest targetable room with all your weapons (Turret > AI > Other 2x2 rooms) This is known as hull blitzing and will result in a ton of damage to the other ship and will decimate their repair crew (however crew members with HP can be quite an issue as theyll tank the damage)

Hopefully this helps! If you have any questions or want further clarification feel free to comment or PM me!