I believe the autocast plugin can do this, i don't have a Google Home speaker myself but it works with my tv box that has a chromecast build in.
There is a big downside to it though, it only works if the device that is executing the autocast action is connected to the same wifi network as the chromecast. Not sure if it is worth the trouble but you could solve this by using an (old) Android phone/tablet that always stays at home, but then you might aswell have that device speak the text directly, that's what i do anyway.
