Obviously, tax planning/estate expenditure will also be driven by how much you wish to leave for heirs.If you do plan to sell, perhaps the time to do it, will be when your engineering income goes to zero to help minimize tax hit (and perhaps that is when you need cash).General retirement wisdom seems to be to keep 1-2 years cash (near cash) buffer of expenses... then you are less likely to have to sell in poor market conditions.I also like the diversity of fairly broad index funds and ETFs (costs can be as low as 0.05%).

I'm a fan of just putting it in S&P 500 index fund, or a robo investor such as Wealthfront.

I would say open a brokerage account and buy $100,000 worth of S&P 500 index fund.   

I'm not super in a hurry to do it again because of how high the transaction costs were and the other investment competing for my dollars is that total stock market index fund which has $0 transaction costs, .04% management fees, a decades-long track record, and all I have to do after buying is...nothing at all.

My number one priority would be liquidity and the "true" passive income of an index funds.

That's a lot different than an index fund.I'd keep it and have a few like yours, but its what I do and would be one of many in a portfolio. 

That is good, but it’s a lot more work and risk than buying an index fund that should average about 10%.

There are a lot of other reasons to refinance in addition to low rates and investing in other properties.For one, if you don't need the money in at least five years, you can invest in an SPYDER index fund or something similar.

It looks at the current rates of other STRs in your region and uses that as an index to adjust your pricing. 

I am using Index Universal Life to fund my real estate investments.