Video export incredibly slow or failure

Photoshop CC 22.3.0

iMac 2019, Big Sur, plenty of memory/disk space

I've got a 6 second video timeline animation. 3 layers with 2 tweens (simple enlarges), total. When I render video at H.264 (or any codec) I get to the 'Exporting Video' dialog box, where the progress bar doesn't move and the cursor spins. Meanwhile, Photoshop immediately writes a a 6 second .mp4 to disk, but the file contains no animation. Sometimes, I can hear the fans on the computer fire up as if it's doing some heavy processing.

I've given Photoshop and Media Encoder full disk access and restarted. Still no luck.