It is difficult to answer without a great deal more info.
If you still want Time Machine to work on the network, then I recommend a Synology router RT2600AC plus USB hard disk. This will work a lot better than your Extreme plus USB drive since Apple never bothered to use USB3.
Another alternative is to plug a NAS.. again Synology is recommended.. into your network with almost any router. I use Asus router which I prefer over the Synology. Asus also apparently supports Time Machine to USB drives.. but my experience was it corrupts itself every 8 weeks or so. In fact Time Machine is not so hot to a Time Capsule these days.. so using a suitable 3rd party like Carbon Copy Cloner is recommended anyway, and should give less issues.
For extend wireless you have more choice than before.
To start with the wireless performance on newer AC routers is far superior to the Extreme and Express combo.
There are extenders but these are becoming superseded by mesh products. Even the routers recommended above have built in mesh option and you can buy another synology or asus that matches it.
If speed is important you cannot beat ethernet wiring plus using WAPs. (Wireless Access Points). Installation is expensive but performance is huge and reliable.
Better end mesh products like Linksys velop or netgear orbi also use high speed wireless backhaul and is superior to cheaper mesh like google wifi.
The dedicated mesh products are better for mesh.. worse for router options and performance and no built in Time Machine or even hard disk to USB.
Give us a list of specs important to you.. including cost. Also idea of your technical ability. Some products are fairly easy to install and others require much more knowledge and experience. Almost nothing matches the Apple Airports for simplicity.