I went this route. Nightmare. The installer software has been removed for Mac and isn't available anywhere. Had to find a laptop running Windows 7 to get it up and running. It also won't be compatible with modern routers. 1/10 can not recommend.
You can get Airport Expresses on Goodwill for $5 to $10, but you need an old machine to set it up.
The current version of Airport Utility will see them in an extended network, and plot them on the network graph, but can't be used for configuration.
It's still there. I have an Airport Extreme that very happily updated itself to support Airplay 2 a few months again
But why wouldn't it work with modern routers? As far as I know, even the newest WiFi routers still support 802.11g clients.