Mind sharing the current components you're using for TB4 -> PCIe Switch -> 826691-B21 ? How are you powering the cage?
Yes, I have a ADT-Link TG3 TB4 to PCI 3.0 4x, then a CEACENT CNS44PE16 PCIE switch.
I was able to confirm the two test Micron NVME U.2 disks work today on my mac directly connected to the u.2 interface from the bridge. I tried the backplane and it didn't work.
I powered the backplane up on the 12v supply with 1,6 and 2,5 as ground, and the PSU went into current limiting until I upped it to 500mA. It then drew about 400 mA with out a disk installed. I installed a disk and got some flashing lights, but nothing on the PCI port. The current draw peaked at 950 mA, and settled to about 880 mA.
I confirmed 5v and 3.3v from on the on board regulators.
The pin 3 on the PSU connector (the only other wire present in the included backplane to 5 pin HP server power cable) has 5.1v on it and it could send no more than 5 mA of current, so it's likely some kinda sense cable.
What i did notice here was significant heat from the board on the lower back side behind where port 7 NVME is. It appears that some part there (a cap perhaps?) is drawing several watts of current from one of the rails.
Are you able to confirm what the backplane draws without anything other than 12 hooked to it?
I'm thinking I may have a defective backplane.