Hi there...you're definitely on the right track with your investment strategy.
Selling your Atlanta property might be a smart move, especially considering your ARM and the potential for rising interest rates. While the property has good cash flow now, locking in your gains and reinvesting could be beneficial in the long run. It's also important to remember that low maintenance costs don't last forever, and that will an increase expense item as time passes. Using the 1031 Exchange is THE way to defer capital gains taxes and use your equity to purchase additional properties.
re: the Pittsburgh market...it can be a good choice. The lower property prices mean you could buy multiple properties, potentially doubling your cash flow. Just make sure to do thorough research and fully update yourself on the changes in the local market to ensure strong rental demand and solid market data.
Diversifying your investments by owning multiple properties spreads the risk and helps maintain cash flow. Multi-family units are particularly good for this, providing consistent rental income and reducing vacancy risks. With your current tenant moving out, it sounds like it could be the perfect time to plan your sale and exchange.