what is a good replacement for an airport express 802.11n

If you want a cheap way to expand your current setup without mesh then Airport Extreme .. either the Gen6 AC version which I am seeing on eBay here (Australia) down hugely in price.. some going for $60 or less. Or lots of the Gen5 A1408 for a couple of dollars. The Gen5 was the best of the older ones and has hugely better wifi than any express.

Let me also encourage you to think about getting the place wired if you own it.

Ethernet is hugely superior to wifi. Bob and I use ethernet to keep old routers running.. and it does a much better job.. and will continue to do so in the future. Wired ethernet to a couple of key locations around the house correctly installed is costly but will last for the life of the house so is a great investment.

Also let me add mesh like TP-Link Deco M5 is really cheap and very easy to setup.

They are well worth a look if you can find them in your part of the world.

We do not use extenders now.. use mesh products.

Some have very small footprint like the google nest or Eero.

Here is a review site..

it covers both wifi6 AX

https://dongknows.com/best-wi-fi-6-mesh-systems/

And older wifi5 standard. i.e. AC.

https://dongknows.com/best-wi-fi-5-mesh-systems/

It sounds like the AirPort Extreme and AirPort Expresses are all 6-7+ years old.....like older....so you really have a couple of options here.

If things are still working OK and you are not in a rush, go ahead and continue using the AirPorts for another year or so.

If you really want to replace the AirPort Expresses, it would not make sense to do so without also replacing the AirPort Extreme.......which is frankly out of date performance and security wise. Go with a new mesh system as LaPastenague recommends, which will far outperform an "extended" system like you have now.

When you install the new mesh system, you won't be using the TP-Link extender though, so you can sell it on Ebay or some other online site.