How many boxes do I need to buy if I mostly care about having premium and maybe BP for the year?

• I will be paying for WoT+ if playing that month and getting 500 gold on most weeks, so "earning" ~2,000 gold per month.
• Premium is 2,500 gold per month or 20,500 gold per year. And you get a few through missions / Twitch drops / etc. so the WoT+ gold will nearly cover that.
• BP each chapter is 2500 gold, so 7500 gold per BP, 22,500 gold per year. But I have the BP tanks I care about, the next I'd get is BZ-58-2 so perhaps I don't care about BP.
• I might be buying some special BP chapters to get the marathon tank if I do the marathons and the tank seems good. So about 11,000 gold there?

Any idea how many boxes to buy and if I should even buy any at all, or just buy gold throughout the year?