FINALLY!
I just installed Sonoma and have tried the new controls, and they work as expected. I found that turning off Center Stage, zooming in and panning down, turning on Portrait Mode with the slider in the middle and turning on Studio Light with the slider slightly to the right gives me exactly the effect I want, and the reactions are a fun addition.
The image quality still sucks, but the added flexibility and the new features add up to enough benefit for me to retire the 5D Mark III I have had mounted on top of my Studio Display.
It wasn't immediately obvious to me where the controls were, I found this helpful:
Right alongside new video conferencing features like reactions, macOS Sonoma gives you Center Stage options to control how you present yourself to the world — if you have the right camera.
appleinsider.com
Too bad I hardly do any videoconferencing now, I could really have used this when I got the Studio Display a year and a half ago.