Hey folks
I've done just a bit of testing with the latest versions of Catalina and the new Music app. Apple has restored the ability to control the new Music App from the Remote iOS App, and in doing so, has enabled Indigo, via the current iTunes Plugin, to now control Music on Catalina as well.
One note... my test was running Indigo Server on Mojave, and the Music App (on a different computer) in the 10.15.2 version of Catalina, but I'm assuming the Indigo Plugin will work when the server is running in Catalina as well.
For me, this is a bit of good news!