If you are worried about lump sum investing in your chosen asset allocation, consider that your risk tolerance may in fact be lower and you should use a more conservative AA
If you keep it all in cash, what would your overall asset allocation be? Say you already have 100k invested in 100% equities and you have a $100k windfall. If you hold off, you are choosing a 50/50 asset allocation. It's ok to keep this cash, but you should know your target and rebalance into equities when they fall.
From what you said about betterment, you seem to think 90/10 is right for you, in which case I say dump it in as long as you can stomach a drop