I would like some advice on this please. My original plan for coming out to my family is to wait till I finish university and have a stable job, being able to provide for myself. Then I would tell them via letter/phone call/person, still not sure. They're Christians with a rather firm belief in The Bible and are mildly homophobic. It would be nice to have their support in life, but if I have to sever ties I can deal with that. Does this sound like a solid-ish plan?
For the most part it is. I obviously don't know your parents but In the event that you think they will react violently, I wouldn't tell them in person. A phone call could most certainly work but just make sure you come out on your own terms and in a way that you're comfortable with.
Yes, contact them when the time is right (when you are no longer relying on them) and in the safest way possible, so phone call or letter would be great, as it gives them time to think about the situation. Good luck!