Compared with "will do", "will be doing" emphasizes more on the state of doing, as if the action is onging. And your thoughts really get into the future world. But I'm not sure if your first sentence is totally unacceptable. BTW, I think at the begining of your sentence, it'd be better to say "If I had known..." instead of "If I knew", as you are talking about an IMAGINED situation. Just my thoughts on this. Im not native tho.

Hi Dolco, I get asked this question often by students and the answer for examples like the ones you gave is that many times, the most natural sounding way of saying something is simply the way it's said most often. Both your examples are grammatically correct, so it is the most common way that sounds most natural to us native speakers. I know that probably doesn't help you with a grammar rule that could help you choose the best version, but this kind of thing is best learned by asking native speakers.