AFAIK, no, you can't resume. Once you leave any streaming source in slacker and come back to it, it will play the next song in the queue.
But one thing I noticed, on my 2016 MS, whenever I come back to it, it will repeat the song that's playing from the beginning, where as on our 2019 MX, it will just continue from where I left it. Not sure if there is a setting somewhere that I'm not seeing.
Don't have a Spotify account, so I can't comment on that.