Of course you should borrow to invest in productive assets! The only question is how much should you borrow.
When I was 14, I learned that a local dentist had paid off the family home, and had then borrowed to buy a rental property. The interest was tax deductible, and the tenant's rent helped pay off the mortgage on the rental property, so he then borrowed again to buy another rental property. He now had two tenants paying off the mortgage on the second rental property. Repeat the process.
That was my introduction to investment. I thought 'just like that'.
In Australia we had, and maybe still have, a managed fund that is leveraged. The rules are that the higher the interest rate, the less the fund borrows to buy stock, and the higher the dividend yield, the more the fund borrows to buy stock. I do not remember the name of the fund.
You can copy this strategy. Other posters have suggested that this is not a good time to buy an ETF, because P/E are high and dividend yields low. But interest rates are low, so I suggest modest borrowings. I suggest a loan that you can readily afford to service. It might be $50K or even $100K. The ETF will pay dividends, which will offset your loan servicing, and the interest is tax deductible. Once you have paid down the loan on the ETF, borrow and buy another ETF. You now have dividends from two ETFs helping pay down the loan on the second ETF.
If there is a large fall on the stock market the value of your ETF will fall, but what will be the effect on dividends? My experience is that companies still pay dividends even if market sentiment turns against them. The question is, can you continue to service the loan? You do not benefit from high stock prices: you benefit from a dividend stream.
My teenage years dentist apparently did not understand the stock market. Otherwise, his strategy was sound.
As Warren Buffett has said, it is hard to go broke betting on the American economy.