So this weird. I have a 2 bay OWC bare drive dock connected via USB-C to the USB-C ports on the back of my Apple Pro Display XDR. I assumed it was a powered hub but even so, drives in the dock (and powered on) will not mount on boot with all 3 1M2 USB4 drives connected and a thunderbolt Envoy Pro SX drive connected to my powered Caldigit Element TB4 hub!
If I disconnect the thunderbolt drive, everything mounts. If I disconnect one of the USB4 drives but leave the thunderbolt drive connected, everything mounts. And to make even weirder, the Pro Display XDR itself is connected from a downstream port on the Caldigit TB4 hub so it shouldn't matter if the hub in the Pro Display is powered or not!
Tried moving the TB3 drive from the Caldigit hub to a downstream TB3 port on the rear of my UAD Satellite (which is daisy chained to the back of the TB3 Apollo) - everything mounts at reboot. And I can connect a second Envoy Pro SX TB3 drive to the now vacant downstream port on the Caldigit and everything is visible with no "too much power" warnings. I'm sure it won't survive reboot though.
Final update: connected the USB3 OWC drive dock to the downstream daisy chain TB3 port on the UAD Satellite instead of the Pro Display, and connected 1 TB3 drive back to the 3rd downstream port of the Caldigit TB4 hub. Everything survives reboot this way, so even though I don't understand what's going on with the hubs, I'm happy enough - 3 USB4 drives, 1 TB3, 1-2 USB3 SSD's on front ports of the Studio and 2 HDD's in the OWC dock, all good to go on reboot.