Other than Auto-Tune, which new inventions in musical instruments/studio technology from the past 25 years have considerably changed the way music sounds?

no, you're exactly right. while the available spectrum has always been there, access to the spectrum has grown enormously, if that makes sense.

Did the fact that GarageBand was bundled with the Mac OS have an influence? I imagine that if every VCR had come with a video camera, we'd have a lot more filmmakers.

Triggered drums indeed changed things in metal, but that revolution was 25 years ago already. Sonically there’s not a lot of difference between 1999 and 2019 production in say death, thrash or power metal, while you can date a 1989 or 1979 album within 2-3 years almost instantly.

You'd think there would be more Ableton-assisted developments in metal, like tuning the guitars down farther than physically possible. But maybe that's too false.