investor of all times said that he would advise someone who didn't have or want to spend the time to learn to put their equity portion in an S&P 500 index fund. There are many choices besides Vanguard. That advice was years ago, before there were many other very low expense index funds. Like the many total U.S. stock market index funds.
James Cloonan, the founder of AAII, in his book, "Investing at Level 3" (I highly recommend the book), recommends the Invesco S&P 500 Equal Weight ETF (RSP) because the equal weighting gives more prominence to the relatively smaller cap stocks amongst the S&P 500 that have traditionally done better than the largest cap stocks.
As for simplicity, nothing is simpler than buy and hold.
I don't know why Vanguard and Schwab, to name two, have such low interest on their settlement funds. Just transfer those to a regular money market funds when the amounts become large enough to worry about. For example, the 3 Vanguard money market funds are yielding between 2.31% and 2.47% for your ready cash.
https://investor.vanguard.com/mutual-funds/list?filterAllAssetClasses=false&filterMoneyMarket=true&filterFiftyThousandAndUp=true&filterLowCostInvestor=true#/mutual-funds/asset-class/month-end-returns
brokers, fund managers, and others make money whether I do or not
Someones are making plenty of money too -- the banks and brokerages -- when you put your money in a money market fund. And you get a pitiful 2 - 2.5 percent or so. Certainly not a good deal for the average Joe or Jane.
The expenses of the Vanguard S&P 500 index fund is 0.04%/year. If the average annual gain is 10%, as it has been historically, you keep the other 9.96%. That's sure better than settling for 2 - 3% on a money market or CD.
The Schwab S&P 500 index fund (SWPPX) and the Fidelity 500 index fund have an even lower 0.02%/year in expenses. You keep the other 9.98% (again given the historic average annual 10% return).
I'm sure the total market index funds, which I consider a better choice for a single fund (because they include small caps) have the same low expense ratios.