I'm afraid there's no way to downgrade on your own; I've seen a few people on Reddit saying that it may be possible to book an appointment at an Apple Store and request a downgrade there, but I don't know if that's still a thing, or if they'll be able to do it for you on the spot. Also, I can confirm that resetting to factory settings does not work.
Two things I've noticed: (1) if, like me, you're precious about your move/exercise/stand streaks, keep an eye on your watch throughout the day: the tracker seems to get stuck at random times, so for the next ~2 weeks (until we get a new update that will bring our Apple Watch Ultras back to "normal") you may see a few gaps in your activity, gaps that may last hours; (2) if you have a Sleep schedule that turns on automatically, you may want to turn that off for now: when it's on, there's a higher likelihood that the sleep tracker will cause the watch to freeze overnight, and in the morning your watch may freeze as it transitions out of the Sleep focus. For now, I'll use a sleep tracker on my phone instead.