@Jon Gorman it sounds like you've done very well without much commercial RE experience so first off congrats on that.
But, assuming you want to keep going and expanding in CRE here are a few suggestions.
You said you based your rents "what was just enough to cover the mortgage and personal loan and all operating expenses". That's not a great strategy longer term because you likely left out things like capex (new roof in 10 years),profit and vacancy.
You should base your rents on comparable rents in the market. The fact that you are 100% occupied suggests to me that you are priced below market. Not a bad thing, just means that you should aggressively raise rents to get them to market as allowed in your lease.
Also, I agree with the comments above that you should try to move to NNN leases to reduce the hassle to you. Let the tenants take care of those expenses especially if that is common practice in the area.
To get there you'll need to get better leases.
To achieve better rents, leases and additional properties, you have two choices 1) Do a lot of research or 2) Find a good commercial broker who specializes in this in your area. I believe #2 is a much better option for you especially since your main gig is as a medical professional.
They can help you evaluate your current property's rents and leases and then find you additional properties to expand. You will likely make more money that way and have more time to concentrate on your medical practice.
You can find them by looking at similar properties for sale or lease on Loopnet or CREXI in your area and talking to commercial brokers in the area. You can also google 'commercial broker in location' to find them. Marcus & Millichap, CBRE, JLL, Colliers, Cushman & wakefield are national names that likely have local offices nearby.