Good example.
However, I think you should sell this paid off one in the possible future when it can be sold for reasonable money, and use the cash to buy a couple of home right now with much better cash flow.
Investing in RE, the important is leverage and cash flow. When the price increased to a level that rent and housing price are total out, it is time to sell and use the cash for better return.
Buying rule is monthly rent is 1% of the price, selling rule will be when the housing price is >160~250 times (you can make your number to determine your comfortable level) of the renting rate.
So, based on above rules you should have sold when it is worth 420k.
Any disagreements? Good lessons learn from hard money!