You can’t completely avoid WiFi - the WiiM home app for one thing runs on your phone and communicates wirelessly to the WiiM device.
Chromecast works by sending a request from your phone to google’s cloud servers to deliver a particular service to a different endpoint I.e. not back to your phone but over the internet to your router and from there to your WiiM device which can be either wired (Ethernet) or wirelessly (WiFi) connected to your router.
Choosing a preset or a URL from the WiiM home app on your phone doesn’t really work much differently - the WiiM Home app tells the WiiM device to request that url from the internet, and its return path to the WiiM is the same i.e. to your router then to the WiiM device either via Ethernet or WiFi.
As I say, quality doesn’t really get impacted no matter which route you take - a network stream url, Chromecast or even Airplay, Ethernet or WiFi, given jazz radio.com only delivers lossy 320kbps at best. I feel your aversion to WiFi is misplaced
A WiiM device isn’t really that different to what you probably do with your current streamer… phone, but it would come via your router and then either over WiFi to your WiiM or over Ethernet if you had the WiiM cabled to your router