ELI5- diversification questions

I like to keep things easy so have put everything into FZROX for myself and kids. Is this a mistake? I see a lot of VTI, VOO. Should I be splitting assets between other low cost funds?

Second question: emergency fund is entirely in a HYSA 3.4%. It’s large (6 figures) because of some income volatility (commission job). Should I move some to Fidelity CMA SPAXX/ SGOV etc or is this largely a wash? I don’t anticipate needing to use it often but as we know the issues that would trigger emergency fund withdrawals are unexpected. I am unclear on the tax benefits of doing this.