Is everyone who is suggesting US listed ETFs investing less than $60,000, or are you at least US person from a tax perspective?
As a US situs asset, SPY should only be considered if you fit one of the above two categories, otherwise non-US situs VUSD listed on the London Exchange is for you.
Both ETFs give you exposure to the US S&P 500 Index, which represents 80% of the total US stock market. The only difference with VTI is you get close to 100% of the total US stock market, including small and mid cap.
As for market timing, good luck with that. As the saying goes... the best time to invest is now. Don’t build up savings in cash and try to time the market. As soon as you have anything to spare (that you don’t need short or medium term), put it to work.
If you haven’t been doing that and have built up cash savings, either invest it all now or if it makes you more comfortable, divide it into 12 equal parts and start dollar cost averaging on a set day each month. Don’t forget to add any additional savings you can scrape together with that as you do.