How to back up without the using AirPort Extreme

A NAS you plug into the network.. Synology works great with Time Machine.

Or you can backup from one network Mac to another if you use High Sierra or later.

Or you use a local disk on each Mac.. plugged into USB or whatever suitable port you have.

Sorry.. NAS is a class of small Network Accessible Storage.. It is like a server but based around a fairly low end dedicated processor much like a router with firmware and RAM sufficient for the task.

Synology is a brand of NAS.. they come in many sizes and styles and price points.. from single disk models to huge enterprise level monsters with 20 disks.

For home use a 2 disk or 4 disk are usually the most popular choices. They are not cheap though.. the basic low end model will cost you $400 by the time you put disks in it.. and well over $1000 for the 4 disk models. I recommend you start with a low end one so you get a feel of what they do.

See the tests here to get idea of prices and what is available.

https://www.smallnetbuilder.com/tools/rankers/nas/view

The Time Capsule was the cheapest available device for Network Time Machine backup.. but it was also fairly limited in that was pretty much all it was designed to do.

You can replace the Time Capsule today with a wireless router that supports backup to a USB 3 hard disk. Synology makes a popular Apple router replacement.. RT2600AC which has almost identical firmware to their NAS range.. and simply plug in a USB3 external hard drive, Mac formatted, and run Time Machine backups.

https://www.macobserver.com/tips/synology-rt2600ac-apple-airport-extreme-replacement/

https://forums.macrumors.com/threads/changing-from-airport-extreme-to-synology-rt2600ac-thoughts.2110024/

They are not without issues.. I have one and it excels at some things and not others.. as does every other router on the market.

Several other routers on the market do offer this functionality.. Netgear and Asus to name two.. but my experience with them is less than fabulous for Time Machine and I would have trouble recommending them.. Synology on the other hand is quite good in both router and Time Machine backup role. Other manufacturers may enter the market.. but you need to remember Apple is still a fairly small segment and making specific routers is very specialised and expensive.

Due to Apple's dropping the AirPort Express, I now have to spend over $1000 on several devices to replace the same processes (WiFi and backup)

The Express was unique.. (well almost.. a near unheard of D-Link product had a go challenging the Express.. it was a total flop).

You can buy ethernet and wireless adapters that will handle network to airplay.. fairly cheaply. Just search ebay for airplay some time. Most are knock offs from chromecast boxes that Chinese add functionality to. But nothing combined router and airplay with digital or analogue output.

At the same time the Express was already way outdated the day Apple released it.

The Gen2 Express in 2012 used low end Atheros processor SoC, very low end N wireless. It fit in the line up mostly as airplay adapter.

Your current Airports and Express are not going to stop working just because Apple has stopped manufacture.. and you can keep using the TC & Express until they fail. Both can work fine combined with routers of other brands..

Apple is driven by profits.. even during Job's era they dropped unprofitable lines.. printers for example. Fact is .. there is too much competition and Apple clearly was not making the margin on airports that was needed to maintain a flow of innovate and cutting edge products. The last series of Express and Airports were on the market for 5-6 years.. an incredibly long time and newly purchased ones should have lifespan of 3-5 years. But the writing has been on the wall for ages.