Wifi Extender
Looking to purchase a Wifi extender that works well with AirPort Extreme
Mac mini (2018)
If they only connect wirelessly wifi extenders can place quite a demand on wifi routers. Since the Airport Extreme is relatively old technology you might see a drop in performance for its connections.
If you are able to run an ethernet cable between the Airport Express and the extender then that is the best configuration.
But for the cost of a wifi extender you might be best replacing the Airport Express with a modern router that might reach the areas with weak wifi.
A tip: in a new router's settings you should be able to change its SSID and password to the old router's values. All your current wifi devices should then connect without issues.