Are you locked into using Airport Extreme or Express to extend the network (i.e., already own them)?
If not, Netgear routers have their own brand of extending units. This could also mean that their brand is more generally WDS-capable, but I have not tried them with AE. In general, I have found their routers very durable, very good signal strength and the new ones are quite easy to set up.
On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.14.x,; iPhones, iPods and iPads galore!