As a first time investor, would you suggest FSMOne RSP for Vanguard S&P 500 or buying ETFs without a RSP?

The former if you are not comfortable with throwing the entirety of your spare cash all at once.

The latter if you want to enter the market ASAP with a sufficiently large capital (otherwise the min trading fees become a large %) while having full control.

Personally, I went with robos first before moving to an online brokerage.​​​

Do you intend to DCA monthly or less frequently? Is your amount huge or not that much?

If you intend to DCA a few hundred a month to buy VOO, FSMOne RSP with minimum fee of USD1 is a pretty good choice.

If you would rather go for a larger sum or purchase less frequently, then perhaps buying directly via TD Ameritrade (no fees) or Tiger Brokers (very low fees) would be a better option. Also if you go this route, you might want to consider buying the Irish-domiciled version, i. e. VUSA or VUSD, to save more on dividend tax. In this case, there are not many cheap options for buying shares on the London stock exchange. I think standard chartered is a better option for this, with a minimum brokerage fee of GBP10.