Between investing directly into a Index Fund (Eg S&P500 Vanguard ETF) vs going through a Robo-Advisor, what would be the main pros and cons?

With a robo advisor you would not only invest into one single equity type. It is some kind of active management, dynamically changing asset class allocation with changing economic ambience. This has it's (managing) price and is fully dependent on the underlying algorithm(s) of the robo, with unpredictable future success of an implemented strategy possibly very much designed on past data.

Passive DIY ETFing with appropriate selection (cheap, large, global diversification, ...) could very probably be at least as successful.