“There are no magic numbers in trend following", written by Norman Fosback wayyy back when. Moving averages (MA) are just lines on a chart based on arbitrarily chosen numbers.
The longer the MA, the less noise and the fewer the number of whipsaws. In return for that benefit, your trade execution will be late and by the time you act, your position may have suffered a sizable loss. A shorter MA will be more sensitive to slight price changes and you'll have a more timely exit. Neither MA length contributes anything toward determining whether price reversal will be small (and then reverse and continue trending upward) or if the downward move will snowball.
Using a moving average as an exit strategy is very similar to using a stop loss order with the difference being that a stop loss order is in the price domain in dollars whereas an exit based on moving averages is based on a derivative of price. Since the MA a derivative, the size of your stop will vary because the distance of current price to the MA is a function of the rate at which past price been changing.
Backtesting this strategy will tell you what has worked in the past (it's curve fitting). Past performance is no guarantee of future. results. The success of any moving average crossover systems is dependent on selecting the right periodicity and that is only known in hindsight.
AFAIC, price is your best indicator. Use trailing stops. If you're concerned about price gaps or 'crashes', you can use options which were designed for the purpose of insurance. Which option strategy is best depends on what your risk tolerance and goals are.