Hello, the qspi flash does work with 2.9.1. Check this out:
Xiao BLE Sense using Mbed LittleFS on QSPI blockdevice - Products & Technology - Seeed Forum (seeedstudio.com)
For the erase, you need to erase at least 4kb (4096), which is the return value of bd.get_erase_size(0). In addition, you need to pre-erase the flash before read/write. Otherwise it is unstable. Good luck.