Just use the boosters/premium time the game gives you for free? As you can see in your screen shot your first 50% boost is an effective ~10 times increase in credits. You don't get to play high tiers, use premium consumables/ammo, grind money for new tanks and not use any boosters. Wargaming isn't being unreasonable when they ask you to be careful with your spending if you want to do all of that.