I have all of my investments in an S&P 500 ETF. I've been wondering - should I diversify my ETFs or go all-in on the S&P 500? In other words, go large cap only, or large + mid (and or small) cap?
You may want to consider reviewing Boglehead wiki link I posted, specially the section about Asset Allocation. If all your investment is in S&P500, you are exposed to US Large Company stocks only. You may want to consider diversifying your portfolio to other asset classes. At minimum, you want to setup 3 ETF/fund portfolio: Total stock market, Investment Bond, and International Equity.
Studies have shown that a 60:40 stock:bond portfolio performs almost as well as all stock portfolio but with lower yearly volatility.
Bogleheads wiki and forum has good information and knowledgeable forum participants.
https://www.bogleheads.org
https://www.bogleheads.org/wiki/Main_Page
In addition, following books will give you good fundamental understanding of why and what of investing.
Your Money or Your Life, Vicki Robbins ...
Millionaire Next Door, Thomas Stanley
A Random Walk Down Wall Street, Burton Malkiel
Personally, 95% of my investment portfolio is in Index Funds/ETF. The other 5% I use as play money that keeps me engaged in investing, finance, and trying out new strategies and investments.