Thinking of investing into ETFs on my own, particularly in the S&P 500. I'm also investing in Syfe's Equity100, so should I just continue with Syfe or do DIY in addition?

Initially you could try both and compare well.

with 0.00 USD trading fees on TD Ameritrade currently DIY will be cheaper.

Consider also MSCI World ETF.

some ideas, on what to avoid, here:

https://seedly.sg/questions/what-is-your-general-investing-philosophy-strategy​​​

Hi there! If you're already invested in Equity100, it could be good to continue staying invested while you explore other ETFs not included within Equity100.

E.g. Equity100 is also invested in the S&P 500 via the iShares Core S&P 500 UCITS ETF (CSPX). As CSPX is domiciled in Ireland, it is more tax efficient compared to a US-domiciled ETF tracking the same index. This is because CSPX incurs a dividend withholding tax of just 15%, rather than the usual tax of 30%.

Ultimately, one of the advantages of investing in Equity100 is that it's professionally managed. There's also no minimum investment and no brokerage charge. So if you want a hassle-free experience, then this portfolio could be right for you.