How much cash do you typically hold onto?
Textbook amount should be 6 months of expenses in an Emergency Fund, and I would recommend a little over 1 month if you are paid monthly in checking/saving combo.
Yes, the market might be due for a correction, but it might also go up to 20,000. I personally felt the same recently and pulled out for a while, but I thought about it further and realized that it is impossible to time. Though I understand the fear well.
The key to this is your ability to repair a portfolio should it go wrong. If you have $100,000 in brokerage and earn $80,000 salary with say... $25,000 in available cash flow (disposable income) if your job was stable you could quite easily go 100% equities regardless of the market, and if we had a 50% drop your salary can divert in to Dollar Cost Average.
However, if you have a very large amount of savings in relation to your income then the ability to repair is reduced - this is why they encourage retirees to shift towards higher bond and cash allocations - the brokerage account will have to provide all the dollar cost averaging itself. So it depends where you are in life...
Paying down your mortgage is always a good thing - though it often isn't the best thing from a pure financial perspective as there is arbitrage involved when looking at mortgage debt (tax favored) in conjunction with fixed income assets - Muni Bonds etc. The Bond route could be a good solution, the only real risk you have there is duration - you need to find Bonds that can be held to maturity to avoid loss of Face Value.