You're right. There are many ways to trade. Some trade deltas. Some trade vol. Some are a mix bag. Some trade forex. Some futures. Some options. Whatever is your forte.
Regarding probability of a certain strike being ITM or not. How does anyone know? It ultimately goes back to price projection and trying to predict what the market will do. What is a high probability trade? If a stock drops 40%, then you can sell puts because it's less likely to fall after a huge gap like that? What is the rationale?
Also, why selling of options? If you can reasonably project where prices may or may not go, why not directly bet on the underlying? More profit that way. I guess the advantage to selling options OTM is larger 'margin of safety' and so it seems 'safer' than betting on underlying, because there is a huge cushion before you get 'into trouble'.
Part of the pricing model with options includes IV or implied volatility with a certain period of time. It doesn't predict direction of price, but gives you a theoretical value of where a stock price will be. When you overlay a normal distribution curve you can apply probabilities to where the stock price will be. 1 standard deviation implies the price will fall within a range 68% of the time. 2 standard deviations implies a price will fall within that range 95% of the time. So for example, a stock trades at 100 and 1 standard deviation equals 10 and two standard deviations equals 20 for a time period of one week. According to theory and normal distributions, the stock will trade between 90 and 110, 68% of the time. And, between 80 and 120, 95% of the time. If you believe these numbers because again they are theoretical based on the options pricing model, one can place high probability trades based on these parameters. If I sell a put that is just outside the two standard deviation mark away (Example: 79 strike price) from its price, I have a 95% chance of that put expiring worthless and a 5% chance of being wrong.
If a stock drops 40% as you mentioned in your example the implied volatility will spike and options become more expensive and the premium you can collect is greater. If the market anticipates a 40% drop, the option will price it in. Traders like you and I however will get into trouble if we are selling options and the option price doesn't reflect the drop and surprises the market. This is why you often hear people say sell options when the IV is high and buy options when the IV is low in anticipation of IV either increasing or decreasing.