I just sold my house and now have 200k+ in the bank. Should I invest it all in an ETF?

What you probably want is a Fee-Only Financial Advisor.

You pay them like a consultant and they recommend the best investments they know of given your objectives and level of fear. Their compensation is not tied to commissions on financial instruments they sell you. Which means, they are not recommending lousy investments just because their salary depends on doing so. Their source of funds is simply the direct consulting fee you pay them.

That said, your ETF plan is not hardly the worst. A good index ETF has an extremely low expense ratio, and that is the surest way to make money - not lose it to fees.

You might also look into what an Endowment is, and how universities manage them. This is invested for very long term growth with little concern for volatility, which is very similar to how a young person should invest for retirement.

If you are new to the world of financial investing, I would suggest using what is called a "systematic investment plan". This comprises investing a fixed sum of money at regular intervals into, for example, a passively managed index fund. For example, you could invest , say, $1000, or any amount of your choosing, every other week into SPY which passively tracks the S&P 500 index. If the price of SPY falls you will get more shares with your $1000 and if it rises, fewer shares. Over a period of one to two years, you will end up dollar cost averaging. This way of investing will ensure that you get exposure to the stock market but at the same will also ensure that you are not putting all of your eggs in one basket at the same time. There are also hundreds of other actively managed funds whose performance you can read about on Fidelity, and decide whether you want to allocate some of your money to any of these funds.

Unlike when you have your money in a bank where its nominal value is more or less preserved but there is no protection against erosion of real value owing to inflation, it is possible that you might lose the nominal and the real value of your investments if the market tanks. But since you would not be putting all of your money at the same time in the market, and also would be buying when the market tanks there are reasonably good chances that over the long run you will do well, and end up beating inflation and increasing the real value of your account. But, finally, it comes down to your risk appetite, and, as someone pointed out, what amount of draw-down in the interim you are able to tolerate. In general, trees tend to grow, and so do businesses and the stock market that comprises them, but there are no guarantees.