This topic has been rolling around for a while so thanks for providing a venue for me to post it!
I also have an Amazon Music account for the Echo only, which is great because my 6 year old can ask for music whenever he wants (as he obviously doesn't have a phone). He likes Johnny Cash and Santana best.
Interestingly, someone on Twitter today commented that they get unofficial covers of songs from YouTube Music too - it wasn't something I had noticed myself, although that doesn't mean I haven't been served them as well.
The algorithm does seem to be very responsive to skips or likes and so that does mean things should keep getting better.