If you cannot access the bootloader or if it is locked you're screwed sadly, your only solution is switching the motherboard if I'm not wrong..

Well the only way to unbrick it for free most likely is having Meizu give you the needed .auth file to use with the SP Flash Tool then :<

Hope someone will notice this post and give you the needed auth file, but someone in another thread has been ask for it with no luck, wish you luck :)