Wow! That's amazing that you have worked up to so many plays. I've done stuff like you described with following the fans of another artist - it works to get more followers on Soundcloud still, but IDK that they are likely to listen to the music later on.
The guest mix thing is smart! Good idea, I hope it keeps working for you.
Thanks, and hopefully something similar can work for you too! Following for follows doesn't necessarily bring in active fans too much, and quite honestly I'm trying to figure out how to increase engagement from my existing fan base. I can get thousands of plays per episode and still only a few comments. One of the causes for that is embedding the show (like on my SteemIt post) or using SoundCloud's RSS feature to pipe it out to iTunes.
Since you're a musician and not a DJ it might make collaboration more difficult... maybe do a monthly "mixtape" sort of thing where you work with other artists to do a compilation and the other artists repost? Your sales pitch to the other artists can be that every artist on the mixtape is reposting it so all those fanbases cross-pollinate. Just spitballin'