IT WORKS! Well part time - but has trouble. Let me explain what the good folks at WiiM Support pointed out.
First, as those on the forum have noted, Chromecast needs to be enabled. Interestingly enough I don't have the option to toggle Chromecast On/Off. It is set to Enabled. When I go to Chromecast Audio > and click on it I get another screen that says Enabled devices WiiM Pro. No option to toggle Chromecast on/off or change that. The next screen from there > shows a toggle switch to "Send crash reports to google". They must get alot of those from me. Regardless I can't toggle Chromecast on/off but fortunately it defaults to enabled. (if anyone can explain why I seem to be only person that has a different screen in the WiiM Pro app for Chromecast enabling I would like to know. I have the most recent version of WiiM Pro app)
Onto the solution. Note: This is only for the purposes of playing audio only from the basic YouTube app - not for playing Spotify, Youtube Music etc.
Firstly change the WiiM Pro to wifi mode (make sure you are not in Bluetooth) Then as Support pointed out, on my Android Samsung S8 phone, I needed to go to SmartView (pull down the drop down menu from the top of the screen). Click on Smartview. A screen should come up showing screen Mirroring and Chromecast devices. WiiM Pro should be listed. Click on WiiM Pro (play sound). This will enable Chromecast to play audio from your phone to the WiiM Pro. Then check again and make sure you are still in wifi and the WiiM Pro hasn't switched you back over to Bluetooth during the process. Now go to the basic Youtube app (not Youtube music). Interestingly enough despite all this process, when you click on the cast icon on the YouTube screen, Chromecast/Pro etc still won't appear. But fear not, just start the Youtube you want to play and the audio of the Youtube app will come through your WiiM Pro, using wifi and Chromecast. (again make sure the WiiM Pro is still on wifi.) This can also be confirmed when you look at the WiiM Pro playback screen, it should show the cast icon at the top of the screen, not the bluetooth icon.
Now the downside. I wanted to play through wifi, not bluetooth because of the better bandwidth and range of wifi. Interestingly enough in wifi mode it keeps dropping the sound unless you stay close to the device and don't move. So much for better distance. It is so bad that everything can be playing along nicely and even though you don't move your phone suddenly it drops off. Sometimes its the WiiM Pro connection that gets dropped off. Sometimes even though the WiiM Pro holds its connection and ou can see from the counter on Youtube screen the You tube audio sound is still playing, no sound comes through. What's with that? Frustrating. I am convinced this is not a WiiM Pro issue, but a chromecast limitation as this same thing used to happen with my original hardware - the Chromecast Audio hardware. But only when I tried to cast a Youtube video's audio. I thought it was a Chromecast Audio hardware issue and hoped WiiM Pro would resolve this. Spotify or any of the other supported apps of course worked fine. Maybe because I am casting the entire app from my phone when I use Youtube vs the phone only being a remote for Spotify etc when the apps are built into the device.
There you have it. It works but holding the device / connection etc is a problem.
