This new Spotify update looks really promising. I can now stream in 'Standalone' mode which is using phone Bluetooth when available, rather than WiFi. Offline mode now allows you to shuffle play which bizarrely you couldn't do previously. And I've stayed logged in all day. Maybe they've finally fixed it?
One thing they still haven't implemented though, which annoys me, is an option to filter for downloaded playlists only and the playlists are still all in a seemingly random order. So, if like me, you've got loads of playlists but only one downloaded to the watch, you have to scroll right through the list until you've found it.