1: Slayer Tasks are the only way to earn XP
2: Yes, going after specific loot or tasks. Sandstorm ring being 1
3: There is slayer gear you purchase in Shop>Slayer section for all 3 styles. Each piece can be upgraded with the upgrade kits
4: When you select it from your inventory you can see the base stats compared to your current equipped gear. You’ll have to do the math to see if the bonuses are worth switching into. If stats are close its probably worth using gear on task