This is way overdue, but I wanted to share my success rather than leave the topic on a cliffhanger! I now have angle control. The one change I can think of is that I’ve switched to using an AS5047P, interfaced to the board using A/B/I. I suspect that the AS5600 either didn’t have enough resolution, output stability (especially during erratic motion), or that there was an issue with the I2C bus (sometimes it would hang requiring a power cycle of the encoder). I’m still using the same magnet, so I don’t think it’s the magnet polarization. I still haven’t checked it with a compass, but I found and ordered them looking for diametrically magnetized magnets, making sure that the description explicitly stated the fact. I have some more magnets coming in that I ordered from a separate supplier (once again making sure they are diametrically magnetized), so I will at least have another opportunity to see if there’s any difference, but regardless, it finally works! Anyway, here’s some video (the Uno is only there to configure the encoder over SPI):
Thanks to both of you (alongside all the others, especially SimpleFOCStudio was very helpful) for your help and for making all of this work freely available.