I know that the general guidance is that emergency funds aren't to be invested, but given our financial security and the fact that we already have a personal e-fund in a safe, non-volatile location, would it still be crazy/unwise to invest this 15k in an S&P500 index fund in a brokerage? 

Purchase price: $275,000 Cash invested: $275,000About this homeBeautiful 2 bedrooms 1 bathroom condo in the convenient location of Rowland Heights, close to bus stop, supermarket, schools, shopping centers etc. newer counter top in kitchen and bathroom. 2 parking spaces and storage room.

I manage and own some STRs. 

Even with a property manager taking a percentage, you’d still see a return, and it might save you a lot of stress managing the property from out of state.

Currently, I am self managing all the operations.

Property management is a "nickle" and "dime" business.  

The asset management piece of investing is not focused on enough, in my opinion. 

Most property managers don't charge any markup.

As you said, this allows you to track LLCs (locations) and properties (classes and/or customers).

If you are someone who has a bigger portfolio that you manage I would suggest appfolio or yardi.