My recommendation would be to replace your router with a modern one, or a mesh system such as Netgear Orbi, depending on the size and complexity of the area you need to cover.
Then, get a dedicated AirPlay 2 adapter for each stereo. I would recommend wired Ethernet for stability and low latency, but WiFi works fine for me where Ethernet is not practical.
The AirPort Express units you already have will actually work nicely as AirPlay 2 adapters if you upgrade them to the latest firmware and disable networking so that they only act as AirPlay receivers and not as WiFi units. There are some caveats as described here:
I have a few A1392 Airport Express units which I bought used to get multi-room simultaneous audio support. I updated them all to 7.8.1 immediately when I received them. In my home office I play from iMac to the AE at the stereo for background music, but when manually switching tracks I often get...
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Unlike just a few years ago, there are now several really nice products to choose from to replace an AirPort Express, depending on your requirements:
Our Audio Adapter with AirPlay 2 is a plug-and-play way to wirelessly stream from your iOS device to any speaker with an optical or 3.5mm input. Shop now.
www.belkin.com
Unveil the brilliance of your music with WiiM Pro and Plus. Embrace versatile audiophile streaming that redefines your listening experience with uncompromising quality
www.wiimhome.com
Unleash the potential of your audio setup with WiiM Mini. Experience the magic of smart, affordable, and effortless upgrades that breathe new life into your beloved audio equipment.
www.wiimhome.com
With Eve Play, stream music via AirPlay to existing amplifiers, active speakers or soundbars conveniently and in high fidelity. This way, your tried-and-tested Hi-Fi system becomes the new star of your multi-room setup.
www.evehome.com
The Belkin is the cheapest no-nonsense option that does what it says on the tin. If you intend to use the optical interface, and only AirPlay 2, this one is just fine. No Ethernet, though.
The WiiM app is really nice, and the Pro version has several high-end features and interfaces, all the streaming services and a pretty good DAC.
I haven’t had a chance to try the Eve Play yet, but I intend to.
You may want to read through this entire thread:
Belkin today announced the launch of the new Belkin Soundform Connect Audio Adapter with AirPlay 2 support, which is designed to turn any traditional stereo speaker into an AirPlay 2-enabled speaker. The Soundform Connect can be connected to any powered speaker or sound system using an...
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