Diversification advice

I am looking at opening a Stocks & Shares ISA with Trading 212, and am not sure whether to invest in an ETF that attempts to mirror something like the S&P 500, where I’m effectively betting that the US economy will perform well in the long run, or a cheap global tracker fund, which should include plenty of US equities, so assuming that I cannot accurately predict long term trends in the global markets, should I opt for one of these instead to spread my bets more effectively?

Given your question, yes.

Some could argue that the revenues of S&P companies are global anyway, so if you buy an S&P fund over a global ETF, you will end up with a similar but potentially much more concentrated portfolio.

I think some global equity trackers such as VWRL hold around 3700 stocks.

Yes, it is good option. Very few stock pickers manage to beat the S&P Performance. Another good EFT I am considering is Health Care Select Sector SPDR® Fund (XLV). But this if you believe there will be growth in health related stocks.

I only have 2 US stocks but have a German one and an Developed Asia one which i hope does well.