Have played more since - the reinstall seems to have completely fixed the issue for me. I'm near the end of Act III now, having completed most quests.
It had gotten to the point where every time we jumped on, within a few minutes we'd see some skipping, until inevitably entire scenes would play with no audible dialogue. I had to start reading the subtitles out loud to my partner so that it didn't feel like we were watching a silent film. It was excruciating. Now everything is snappy like it was when we first started playing. No delays, no skipped or dropped audio, the game finally feels cinematic again. Woohoo!
I'd highly recommend reinstalling the game if you ever run into this issue on PS5.
