I have not voted, because my reply would depend on my age and circumstances.
I would pay down the mortgage to $200K, then invest the balance of the money into share funds, domestic and international, and property funds. I understand that mortgage interest for a private house is tax deductible in the US - it is not in Australia, where I live. A tax deductible mortgage is effectively an investment loan to allow another $200K to go into investment.
The background for this goes back to 1968. Alan Shepherd was at that time the largest individual investor in BHP, the giant Australian resources company. I mean individual investor as opposed to other companies investing in BHP. Broken Hill Proprietary, known then and now as a 'mums and dads' investment. Alan was interviewed on TV and said that in the fifties he had inherited 30,000 Australian pounds, (the currency was not decimal in the fifties), and he had borrowed another 50K pounds and dropped the combined 80K pounds into BHP. BHP grew, and paid more and more dividends, which it still does. In the fifties, a thousand pounds was about the average annual income. I imagine that Alan had an independent source of income to service the debt, but as BHP grew, the debt became easier to service and pay off.
Such a huge borrowing is absurdly risky, but some more modest borrowing is still a good idea.