1. When I connected my iPad Pro M1 to the Mac via a 10Gbps rated USB-C/C cable, it only connected at USB 2.0 speeds (480Mbps). However, when I used a USB-C to USB-A adapter, then a USB-A to USB-C cable to my iPad, it connected at 10Gbps. My suspicion is that when it has the full USB-C wiring on both ends, it might try to establish a Thunderbolt 4 connection, fail, and just fall back to USB 2.0, rather than trying USB 3.x?
2. On a slightly unrelated note: I found the USB-A ports (or rather, their controller) to be really unstable when using a hub, everything works fine when connecting devices directly to the Mac, but with any of the hubs I tried, I saw USB devices becoming unresponsive or resetting over time. A similar setup worked fine on my Windows PC. The most unstable was a simple (i.e. 3 USB-A ports and Ethernet) USB-C hub which I tested with an adapter ("proper" one with VL160 chip); for fun I also tested it with an ungodly chain of almost every USB-C/A adapter I had on the USB-C/TB ports, where it worked fine.
Having to do a bunch of troubleshooting to set up my USB peripherals on the Mac was a bit unexpected, at least the USB-C/TB ports seem to work well (other than that iPad weirdness), so I'm happy I went with the Pro model for the additional two ports.
(Note: While I didn't really approach this too methodically, I believe I tried enough combinations of cabling and hubs to exclude a simple issue like a faulty cable.)