I have replaced my Chromecast Audio with Raspberry Pi with a Snapcast client. On the Snapcast server side I run a DLNA renderer ("gmrender resurrect"). This allows me to cast audio synchronously to multiple speakers using the Android app Bubble UPNP. Basically, instead of clicking the cast button in an app, I select share, then Bubble UPNP. One advantage to this is that I can "cast" Youtube and get the audio, which Chromecast audio does not support.

Btw, Snapcast works great.

You can use a small computer and remote control it somehow.

Or use a Chromecast Ultra and connect to the hi-fi with a hdmi to audio (spdif, 3.5", ...) or hdmi to vga+audio (and don't bother connect the vga to anything)

I have a Google Home mini connected via Bluetooth to my amp, with 25 year old Gale speakers. That works for casting, and of course directly controlling by voice. I previously tried connecting an old Echo Dot via the wired connection, but the quality was a bit rubbish. I'm guessing a crappy DAC in the Echo or something. The sound via Bluetooth is great though.