Hi @harrylnorris,
Today I mounted a SanDisk Extreme 64 Gb Sd in the posted USB dongle for uSD cards, with a freshly flashed latest Stable ZynthianOS (on Mac). I manually modified the boot order (sudo rpi-eeprom-config --edit) with the USB - SD - nvme sequence (BOOT_ORDER=0xf614), connected the USB dongle to the Pi5, rebooted and it worked just fine at the first attempt.
I confirm that the posted uSD carrier (PNY) affords remarkably good data transfer rates. Context: there is no SD card in the internal slot of my Pi5 metal case, since I use the USB dongle with Vangelis to override at startup the other system (Oram) installed on nvme, and prospectively also for troubleshooting or general operations with Raspberry OS.
I hope it helps,
Regards
PS Let us follow-up with this aspect of the discussion in the thread where it possibly belongs: Boot from USB?