MBED + STM32F446RE triggers HARD FAULT when trying to write to flash
I will answer to my own question. I am using MBED as environment and I missed this page that basically explains two of my problems.
The MPU is pre-configured by the framework to disable both writing in ROM and executing of RAM. You need to declare explicitly if you want it to be possible using ScopedRomWriteLock or ScopedRamExecutionLock
https://os.mbed.com/docs/mbed-os/v6.4/apis/mpu-management.html