What is the follow on to airport router

I am a former network engineer with $300 to spend

I am willing to spend to get something that will be long term

And you don't have your house wired with cat6.. ??? Amazing.. as a network person I have had the house wired with 10base2 since we bought it nearly 30 years ago.. although it has taken me a while to upgrade to Cat6. Long term nothing but nothing beats ethernet.. wireless won't even touch it. All your wireless gear will need replacement every few years.. Cat6 should last out 20 years.. with cheap upgrades to switches and wireless access points.

If you don't want to go wired.. which is fair enough in circumstances where you don't own the place or build is solid concrete and brick.. then I would go the Netgear Orbi RBK50 (2 units) and you might need an additional satellite. Or you can buy the RBK23 set which is a lot slower.. but will still be a big improvement over the old Extreme and Express and does come with 3 units.

But that does not give you Time Machine backup.. and there is no way you can do that in $300 anyway.

The cheapest alternative with TM is the Synology RT2600AC. It is very sophisticated router and your network background will make it easy. You can use the existing USB drive most likely with the RT2600AC.. you will need to wipe the drive and format it correctly for the Synology.. so if you don't want to lose your existing backup then budget in another 4TB or so USB drive.

For the time being the Extreme could still work just for backup.. A better alternative for the real DIYer is Gen3 A1355 or Gen4 A1409 Time Capsule .. last of the pizza box models.. these are really cheap second hand.. but you might need to replace the disk or power supply.. or even fix the board.. all doable. The internal disk is much faster than USB2 on Extreme.

If wireless is not good enough then you can get the mesh unit. MR2200AC which is a full router but can run as mesh satellite.. or even hunt for a second hand Synology router.. I think the firmware has been upgraded on all of them to support mesh. But mesh performance on Orbi will beat the daylights out of it. So you need to balance out your need.

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.