OK, I understand, that’s very clear.
So, to summarize:
1/ I switch to fastboot mode and unlock the critical partitions:
fastboot flashing unlock_critical
2/ I modify the flash_fp4_factory.command file and change the value of REBOOT_TO_BOOTLOADER from false to true.
3/ I run the checksum (sha256) on the new flash_fp4_factory.command file and replace the old checksum value in the SHA256SUMS file.
4/ I start installing the stock ROM.
5/ At the end of the installation, the phone remains in fastboot mode and I check that unlock ability has the value 1:
fastboot flashing get_unlock_ability
6/ If the value is 1, then I launch the bootloader lock:
fastboot flashing lock
7/ Once this is done, I restart the system in Android.
Please let me know if I made any mistakes.
I will keep you informed of the process.
In any case, thank you very much for your valuable help.