Look, I don’t believe that asking people on internet is the best place to ask, the majority of traders lose money (similar failure ration applies to this forum) and so
be very careful with any advice. 
Nevertheless, consider the following:
What if you invest at the wrong time? The economy is not that good, the government is printing money and created massive debts and that cannot go for ever.
Or Oil / Energy ETF?
Or agriculture ETF?
I'm NOT suggesting that you invest in any of these, I have no idea where the market is going to be tomorrow, let alone in few decades. Nevertheless, putting it all into extended index after years of money printing seems like bit of a gamble at this period of time.
Therefore, maybe some diversification could be considered to reduce the risk.
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Good points;
Dave Ramsey likes it spread across 4 funds+ has in the past bought SPY also.
$300 ,000 would enable one to buy $1,300+/ worth of business books/investing\trading newspapers; say its now $298,688
OLD Stock Traders Almanac has some good numbers\but smaller numbers due to less average compounding, smaller Roth, inputs ,2001edition .
Dave Ramsey says stock market average average =11%;
so get 12% .Start with $2,000per month for 25 years ,Roth or $298,688 start = same start total, today+ to simplify ,end of2023 year=$298,688.
Add 10 years to both , same total [x12%]=$966,926.
Add 15 years to both, same total [x 12% ]=$5,376,041.
But say someone never did well in business, never even tried real estate + rented all his or her life\oops.
But did super well in stocks average, but less than Peter Lynch Fidelity average of 29.2%.
$2,000 per year Roth, say 20% average, 7 figures in 25 years .
20% compounds so well ,super well/ do one's own research on that> 7 figures.
I like business + some IBD reading + RE alsoLOL

