But couldn't I (theoretically) just use e-Trade or an online Fidelity account to invest in mutual funds short-term and take my chances? Or would the buy/sell fees on that kind of amount be too high to make short-term gains worth it? I'm not averse to risk and I don't need exactly 300K. The way the market's been going I feel stupid sitting it out.
Of course you could, if you're willing to be extremely flexible on timing your withdrawal.
Depending on your broker, you could pretty easily find S&P 500 style index funds that trade commission free (Vanguard and Schwab are two brokers that I know of that offer such ETFs), so commission drag shouldn't be a worry.
But if you absolutely have a one-year hard line on your need for some portion of that 300K, there's no guarantees that it will be worth even that much on the date you want to pull your money out.
What would you do if the market takes a 50% nosedive in the next twelve months? It probably won't, but it could. Lots of people have drowned camping too near a river that
probably won't flood. Would you settle for 150K loss to salvage whatever cash you can? Or would you be willing to wait the storm out for a year, two, three, or more, for a full recovery?
Only you know how critical the one-year time frame is, and how important capital preservation is to you within that time frame.
If your timing is inflexible, you could put your minimum cash needed into treasuries, or a money market fund, etc., and put the rest in an index fund.
If you want maximum potential returns, you have to give up control of timing.
If you want maximum control over timing, you have to settle for lower returns.
You can construct a portfolio that splits the difference between these two extremes according to whatever ratio you care to impose, but there is no free lunch in any scenario that doesn't rely on luck.