I enjoy seeing the enthusiasm of the next generation and if I can give them a little bit of hope & direction in how they can change their future and can learn to invest and can learn to help others, I call that a good use of my time.   

Once refinanced it is estimated that the new mortgage will be $5,000/month.Is it worth it to cash-out refinance at 75% to start investing in other properties to generate cash flow properties?

This will allow the building to still generate revenue while you methodically get everything up to market. 

For reference, it would be a roughly $300k property that would generate ~$2,250 in gross monthly rent.

There are many other investors who would buy properties that would break even with one unit not generating income all day long in that area, and they'd offer cash and more than you appear to currently willing to offer.

My goal is to generate another stream of income for retirement and am not interested in amassing a multi-million dollar real estate empire.

It’s an active trade generative ordinary income because one is selling inventory, in this case real estate.

Later on, you can start playing with off-market deal generation via wholesalers or actually contacting homeowners directly. 

To decided if the property would generate cash flow. 

@Phillip RosinA vent is required because water heaters generate a ton of heat.