The Micro works well, depending on the device. On my PocketBook, I pair it in HID/Keyboard mode, and I can map every button to do something in stock KOReader.
The 8BitDo Ultimate app will remap the various buttons. But you need to put it in HID/Keyboard mode.
The Zero 2 isn't as straightforward. You can pair it in HID mode. But I can't find any documentation on keys the buttons map to.