Finding reliable is only half the battle; the other half is executing the flash correctly. Begin by backing up your data, installing proper SPD drivers, and sourcing a firmware package that matches your hardware version. If you are not comfortable with test points and bootloaders, consider taking the device to a repair shop that specializes in Spreadtrum-based feature phones.
Some carrier-locked variants have a flash protection password. The generic passwords are: Firmware ZTE Blade L220