Big gains aren't a crash, although April and November 2020 certainly looked dramatic. Since 2012, according to Yahoo Finance, those are the two best months for the S&P 500, gaining +13% and +11% respectively.
Which is another beauty of rolling more frequently. Say you do sell the $373 put, 45 dte today, and the next 30 days the S&P has a killer month, increasing 15%. Your put will fall so far OTM that it'll decrease in value significantly. You can continuously roll them out and up, increasing your profit faster. Now, each time you roll it comes at a cost. Commissions for one, but you're increasing your exposure to future market crashes, especially in the event of whipsaws.
I might have gotten a confused view on delta online, as your explanation is much more useful in a concrete way. Using the puts at $260 (9%) and $280 (12%) deltas also paints a more complete picture: 9% the entire spread is covered, and 3% of the time the spread is partially covered.
Delta is actually much more complicated than my very simplistic analysis, and it can be used in many other ways. But at it's basic premise delta can be used (i) to explain how much profit/loss you get based on a change in the underlying (it's main function) (i.e. a 25 delta put will lose $0.25 in value for each $1 the underlying increases, and vice versa), and (ii) the odds (roughly) the option will be ITM at the expiration date. All of that is based on the implied volatility, and a number of other factors. But thats why I can often make trades based on the delta alone.
There's still a gap between understanding delta and using it: I don't know where you go online to look it up. I've been using Yahoo Finance's option chain, and an app, and Vanguard's ... none of which display delta. Is there a free website where I can find it online?
Only look at the deltas calculated by the broker you plan on using. Deltas will vary from one location to another. You can often calculate it different ways (as all of the greeks are derivatives, they calculate based on each other in circular fashion). If I look the delta up on ToS and IBKR, I'll often get two different numbers. Not crazy different, but enough. I'd never trust Yahoo or Google to place a trade off of.
I'm not familiar with Vanguard for options. I've only ever used ToS or IBKR. But I know Vanguard will display the deltas. It's often a setting you have to change. You can set most brokers to display as much or as little information as you'd like (Bid/Ask, Last Price, Open Interest, and Delta is what I like to see, some people like to look at the Theta, IV, Gamma, Rho, Intrinsic Value, Volume . . . you get the idea). Try messing around with the settings, and if you can't figure it out call Vanguard up.