Instead of jumping into an expensive Long Island market with negative cash flow, consider investing in a Texas market like San Antonio, Houston, or DFW suburbs—these areas offer strong rental demand, lower property taxes than NY, and better cash flow potential.

That person will be able to tell you what you can do and following that plan will give you peace of mind about your further investing strategy.

Keep this to a single page.

SFH/LTR turn-key is a great way to invest. 

Thanks to everything I’ve learned from the podcasts, books, and this community, my wife and I currently own:A triplex in MaineA single-family rental in San AntonioA single-family primary residence in Las Cruces, NM, which will become a rental when we move in a few monthsFor context, I’m an active-duty Army officer with about 12 years left until retirement, and my wife works for a university in Boston.

Live under your means and start thinking about investing in Real Estate.In your position the thought of finding a duplex to house hack, live in one side and rent the other. 

A 1031 exchange is for selling an investment property then buying another investment property. 

@Rene Hosman You can read about H.S.A. investing in my new BP book - Self-Directed IRA Investing: A BiggerPockets Guide - www.BiggerPockets.com/SDIRA

I've used the search feature on this forum but didn't have much luck.Are there any US investors, hard money lenders, or groups that invest in flips outside of the United States, specifically in Europe (Netherlands, Spain, etc.)?

Until you build trust with a team or contact you should view every property esp a MF that is a larger investment.