The MB20 is basically a Bluetooth receiver, correct? The Wiim Pro Plus can be set to input from Bluetooth or to output to Bluetooth. I have never used either so I can't comment on how well they work. Instead of using Bluetooth, you can use the cast option from the YouTube music app to the Wiim Pro Plus. That is what I do when I listen to YouTube music. That way you don't need to mess with the input settings and can still use the other streaming apps like Spotify, Qobuz, Amazon etc. if you ever want to.
The Wiim Pro Plus is a great streamer with a very good DAC too.
I just fired up the YouTube music app and clicked on the cast option on the top right. It is playing to the Wiim now without needing to turn on Bluetooth input.