I think the word "risk-free" and securities have no place in the same sentence. 😉
Investments in securities are subject to a certain amount of risk, but with $SPYI you have everything in one product - from industrialized nations, emerging markets and small caps. The ETF takes care of the balancing and structure for you.
You can think about other assets (bonds, gold, crypto) as diversification.
Every security you pick up (whether individual security or ETF) is already included in the ACWI and you then overweight it. It doesn't have to be bad, but it doesn't diversify you any further.