An update.
I just performed a hard reset of the Mu-so (haven’t tried the Qb yet but will later). Pin in the hole on the side for ten seconds until the light changes colour.
During the setup process, it specifically asked about confirming agreement with T&C’s for Google Cast. I clicked skip.
I presume this has now disabled Google Cast. Once set up, Spotify Connect is available again, with lossless mode.
Obviously this isn’t a solution for anyone who uses Google Cast on their Mu-so, but it will be fine for me (assuming it works long term!)