i would recommend a
Salsa Fargo for gravel and dirt road mtn biking. you can fit these bikes with a suspension fork, or you can run it with a carbon or steel fork. a new one is in your price range, but they have been around for a while, so you may be able to find a used one on eBay or CL.
i don't think you would benefit from smaller wheels if you plan to avoid singletrack. you will get a relatively smoother ride on the 29er wagon wheels. you might prefer steel over alum for the same reason.
a gravel bike that fits larger volume tires (40mm or more) would be a valid alternative if you are disinterested in front suspension. but in that scenario, i would still rec a rigid mtn bike for the larger tire volume.
at age 78, i would think you'd want the widest gear range possible. a double, or even a triple crank, would be the way to go.