What I want is: 1) desktop/tablet and phone message delivery, with sane notifications and reliability, 2) doesn't feed all my messages to an ad company, 3) works on my non-Apple devices (otherwise iMessage would be entirely sufficient), and 4) good enough that I can get people to switch (or transparently uses SMS, so it doesn't matter).

Signal fails 1 (the desktop app is pretty bad) and 4 (too many little problems, others won't switch). I'm starting to think Slack, of all things, might be my best solution. Really, I just want ICQ with smart phone/desktop notifications, and picture/video embedding, which doesn't seem like it should be a thing I ought to have any difficulty whatsoever tracking down in 2017.