Can I just buy S&P 500 etf SPY?

The current framework suggests using a robo advisor and switching out to a SPY once the portfolio hits 0.8% in fees.

Which you can do after 1 year of $600 a month.

That way you minimize your cost per transaction for both broker and robo advisors.

Not really advisable cos:

1) There's 30% dividend witholding tax which is going to eat into your returns

2) DCA every month via a broker is going to cost you alottt which doesnt make sense

You can try a similar etf CSPX (Accumulating) or VUSD (distributing) that's domiciled in Ireland to reduce your WHT to 15% instead and preferably do a lump sum investment into it. If you like, you can top up into the etf say once every 3 or 6 months instead of every month, to reduce the brokerage fees.

https://newacademyoffinance.com/best-tax-effici...

This article explains very clearly the witholding tax issues. Alternatively, you can try robos, though in the long run, fees will be quite significant as your portfolio grows.