Seriously this needs to be fixed! I got a free google home mini with the spotify promotion and now I can't even do this as intended. It's really sad because now I'm just considering switching to google play. How did that free google home mini promotion work out for you, spotify? Infuriating...
What I've done is use a Chromecast audio into the Line-In of my soundcard and then in the sound settings set the Line-In as default and then in the Line-In properties set "Listen to this device" so what comes in is output to the PC speakers.
I've named the Chromecast audio as PC so now I can say "Hey Google play music on PC" and comes through my PC speakers.
If you can't get hold of a Chromecast audio you could use a normal Chromecast and use an HDMI audio extractor to get analog audio. Also in regards to setting the default sound input to Line-In if you use a microphone for Skype/Duo etc you can select what to use as a microphone in the settings for these apps which overrides the PC default.