I think a more interesting problem is not how you can differentiate a spambot with a 'non-spam' bot. I've seen some bots that are really creative and fun on Twitter. I guess it's not really hard to add it to a spam detection ML model
Non-spam bots generally don't follow each other or link to external websites. (I'm also the author of one of the more popular image bots https://twitter.com/a_quilt_bot)
