Q1: 5000k if you're using 6000k and dont like it
Q2: Every bulb needs one ballast, so if your ballast are powering your low beams you need ballasts for your fog lights
Q3: There is no "specific" ballast that will magically make your bulbs brighter. Try ddmtuning.com
Q4: Since you have Depo headlights with the LED rings, the angel eyes WILL NOT change. Depo headlights have their own powered custom angel eye rings. IF you want to change the bulbs you need to get OEM headlights with Angel eyes so you can swap bulbs
And for your other thread asking about D1S, you can use D1S bulbs since you dont have OEM Xenons. Depo housings use the same bulbs has OEM halogen housings, which are H7.