Regarding the mixing option on Apple devices... that was initially released only supporting purchased or imported songs. For Apple music subscription songs, the playback interface from Apple is very limited and required a more complex approach. I think I have most of the kinks worked out on that and support for mixing all song types was released today 3/20/19. I use phased release with updates over 7 days, so you may need to search for the app in the appstore and manually get the update if you need it quicker.
The only known issue with the mixing feature is muting of the announcement on a small number of bluetooth devices combined with iOS 12. The feature is not workable with those devices unless you use a long aux cable rather than bluetooth.
