Let them pay themselves off in 20 years and then have 4 to 5k in net income to supplement their retirement.

Understandably, most would recommend setting aside more than that, however, I work full-time and am committed to supplementing my investment with my W2 income as well.

In about three years I plan on retiring from the Reserves (I will still keep my full time job) and want a way to supplement that monthly income.

You can try to find a handyman (maybe on Craigslist) who is willing to walk you through the steps and let you help or even do the project with his guidance so you learn, though I'd still supplement with YouTube to make sure he's not missing something or doing something wrong. 

I do know that I’m looking to supplement/replace my income with cash flow at some point in the future but am open to anything that makes sense.

Since I’m now at a point in life where the live-in flip model is too burdensome on the family, I’m looking for advice and to network with the BP community to find new investment opportunities and (hopefully) start building supplemental revenue streams.I look forward to making new connections.

The majority of the people I've discussed this supplemental business with have had some of the typical negative responses full of fear and doubt.  

There is no benefit or reason for you to personally supplement any tenants rent.

IUt's not my real estate investing strategy, I am looking to use ti as supplementation for putting my funds to use, especially when I don't have enough yet to purchase a property.

One of my smaller units has electric heat but a gas fireplace to supplement that really knocks the utility bill down for the tenant.