need advice: Investing in ETFs/stocks, and investing in non-USD currency.

Context:I am 19yrs and I invested about 8k usd equivalent (more on that later) in etf/stock where I made some good pocket money (though I won't cash out). For etf, I invest about 3k usd in VTI only. My main currency is yen (jpy). So my questions are:

1. Should I switch to investing in dollars? While the yen is weaker, I don't think years down the line, the yen would become weaker. But is it worth it to buy in dollars?

2. Not knowledgeable enough but it seems like Japan have there own VTI/VOO called emaxis slim S&P500. Should I still invest in VTI?

3. VTI and chill?