Your question about risk is probably one only you can answer. I see that you understand that you are not connecting and know that, if you miss the second flight that whole ticket will be cancelled and you'll have to buy a return ticket at walk-up prices. There is no what to build a risk model, because you can't meaningfully attribute a likelihood of cancellation or significant delay of the incoming flight. Nor can you predict weather.
So what you have to do is accept that there will either be outcome A (one ticket) which has a fixed cost of EUR 220 per person, or outcome B (separate tickets) which has a cost of either EUR 158 or EUR 158 + EUR 150 (? I'm guessing at a possible walk-up fare ATH-JTR). Personally, for the sake of less stress, I would book this as a single ticket.