I'd say that you ran a mildly smart option strategy because:
Apart from the above, the reason that you succeeded was that for the most part, 2019 was an up trending year. Bullish credit spreads do well in that kind of environment. The secondary reason is that you picked quality underlyings that cooperated (Blue Chips tend to track the market) and picked few dogs that went into the crapper. Otherwise, you wouldn't have done as well as you did.
To survive a tail risk wipe out like this year would require owning negative delta:
- You could achieve that with a mix of bullish and bearish positions.
- You could ratio your spreads, buying slightly more long puts than short.
- You could defend positions, rolling the long puts down and with each roll, slightly increase the number of long puts, turning the position into a backspread. This did quite nicely in March though the massive increase in implied volatility made this harder later in the month.
- You could flip from bullish to bearish verticals due to market conditions but this one is far easier said than done given the speed of March's descent.
- You could buy some OTM index puts as a global hedge.
Regarding the last one, due to the market run up, for the past few years I have purchased one year or longer SPY LEAP verticals with the long leg 10% OTM and the short leg 20% OTM as a portfolio hedge. Cost has been approximately 1.5% or so of the principal. With a few leg out and ins of the short leg during the year, the total cost was reduced to under 1%. Despite this, it was a total waste of money in 2018 and 2019.
Their purpose was to hedge a chunk of my long equity exposure. Toward the end of the year the short leg tends to be near worthless and I cover it, ending up with decayed long puts. Several months before they expire (before the accelerated last months of theta decay), I roll them into the next year's new protective vertical. Needless to say, those long puts paid off quite nicely this year, massively reducing my equity losses.
Per the answer by RWP, you could delve deep into statistical analysis and derive a precise probability based on some period of historical data. It's just my opinion but I think that's a bit of a wild goose chase because it's based on data from previous months and a tail risk event like March doesn't care about your probability number. Your survival will be based on the choice of strategy(ies) as I listed above as well as your ability to manage and adjust your positions or shut them down to cut your losses (active risk management).
Lastly, some suggest that the delta of an option provides a loose approximation of the probability that the option will finish ITM. Probability Of Touch is about double that. I'm not too keen on this because implied volatility affects delta, more so when OTM so such a probability number can fluctuate significantly. Ball park number? Yes. Accurate. Methinks not.