Chromecast Audio's Digital Output is LPCM, not AAC, and should work with any DAC. I have not heard of a DAC that it was not compatible with as long as it has an Optical Toslink Input.
I'm dumbfounded that nobody has done a quality add-on Spotify Connect receiver yet. I have had a Yamaha receiver with it built in for years now -- would be better for 44.1 than Goog-A.
Coincidentally I had a conversation today with the owner/founder of a company that makes Wi-Fi Speakers that have the DTS Play-Fi System ( a CCA competitor) built in. He told me that whether you are using a Play-Fi compatible product, or an Amazon Echo Product, or CCA, they all use the same Spotify Connect system, not their own systems, when playing Spotify. That is why you do not have to fire up the Play-Fi App, or Google Home, or the Alexa App to play Spotify on your wireless speakers. Just fire up the Spotify App and you see the speakers under available devices. The Google Home/Alexa/Play-Fi Software literally just kicks you over to the Spotify System. (I think Sonos may work differently and have more total integration, but the others don't).
SO, there would be little purpose to creating a Spotify ONLY add-on receiver when these other systems all do the same thing...but with the benefits of additional streaming sources. I own several speakers that use the Linkplay (also called Audiocast) system, and some of their Add on receivers have, like Chromecast, Optical Digital Outputs. The limitation is that the Spotify system is not designed to be super high resolution (like say DSD), but rather universally compatible. So, I am not sure there would be a sonic benefit to creating a $500 audiophile Spotify add on, when the signal itself is so limited.
That said, there are $500 "Audiophile" Bluetooth only receivers. IMHO that's ridiculous, no matter how great the receiver is, Bluetooth signals (at least until the new Bluetooth formats released in the last year or so) have been so compressed it could not possibly sound as good as a $35 CCA. Same reason you do not see to many Audiophile AM radios. The limitation is the signal, not the decoding receiver.