Hi @jcv,
I also have balenaOS on Pi5 booting from NVMe. Using the official M.2 HAT+. As long as your Pi5 EEPROM version is 5 June 2024 or newer, you may not need to do anything special e.g. you don’t need to change the boot order. I just flashed directly to NVMe, put it in the HAT+, and booted.
Earlier I booted a Pi5 from a Pineberry HatDrive Bottom. That was in January and I did have to update the EEPROM to 1) enable PCIe, 2) add PCIe (NVMe) to the boot order. I used Pi OS on an SD card to update the EEPROM. That EEPROM updating may no longer be necessary for the HatDrive.