I’ve more or less solved the crashing issue with Stretch- it’s the fault of the dmix ALSA plugin it appears- so I just need to make sure it works and drop a new asound.conf into the Pi VU Meter repo.
As for crackle, I don’t know if it’s possible to eliminate completely but you may have luck with a USB WiFi adaptor if you’ve got one sitting around to test with. I know it sounds absurd using an adaptor with a Pi that already has built-in WiFi, but the onboard WiFi is apparently not terribly good. My own experiences with Parsec and 1080p video streaming seem to back this up, but it could be confirmation bias!
If I ever get the time, I may conduct some objective tests to confirm or deny this.
The README on Shairport-Sync’s GitHub goes into far more detail than I could about what configuration options to use. I believe it’s possible certain options may make a difference to performance, but I haven’t explored them in any depth.