If you have $2k of speakers, I assume you have a receiver that you use to connect it to your TV and other audio producing sources. You simply plug a normal Chromecast in said receiver, and not only you can route music, you can also route videos from Youtube, Netflix, Plex and thousands of other sources to it.

Get a cast-enabled amplifier then? Sony's receivers cast built-in, for example.

But you realize how extremely niche your problem is, right? That there clearly aren't enough of people in your particularly situation who also want a dongle that haven't already bought one?

I have a top of the line amp, why do I have to get a "cast enabled" one? I am not sure what the last sentence meant. I already have the dongles, I'm just sad they are no longer going to make them.