Edited by XRed_CubeX at 2023-04-06 17:19

I am not responsible for any brick.
I tested this guide myself and decided to share it.
Make a backup before this guide because you will lose your data!
The unlock method has been updated and now you can unlock the bootloader more easily without the risk of bricking after an OTA and without changing the bootloader.
Now you can safely unlock the bootloader using mtkclient.
Link: https://github.com/bkerler/mtkclient
TWRP can be installed via mtkclient or manually from rooted Android.
TWRP Download link: https://github.com/XRedCubeX/twrp_meizu_m5c/releases
IF you unlocked the bootloader with the old method you can follow these steps:
1) Download the attached package.
2) Download OTA zip from flyme, link: http://www.flymeos.com/firmwarelist?modelId=70&type=1
3) Download the latest version of SP Flash Tool.
4) Download MTKClient and follow the instructions to install it.
5) Power off the phone and keep unplugged it from PC.
6) Open SP Flash Tool, select the downloaded package and select all partitions, then press the download button, don't plug phone for now.
7) Boot to recovery with vol +, plug the phone on PC, depending on recovery, TWRP or stock, use adb sideload or put update.zip in internal memory and install the zip.
8) Let the phone reboot or reboot it manually.
9) Now start the flash from SP Flash tool automatically, let it finish.
10) After the flash, unplug the phone and reboot into recovery.
11) Paste the update.zip in the phone and flash it.
12) After reboot, let your phone reboot into android and power off.
13) You can follow the guide to unlock bootloader with mtkclient.

FAQ:

A) But will I lose the data with this guide?
Yes.
Make sure to back up your data in any case!

B) But this guide is a bit different than usual, where is "fastboot oem unlock"? Or "fastboot flashing unlock"?
Meizu has not implemented an unlock command and in this case to unlock the bootloader we have to use a tool that makes us change the unlock status.

C) Are there Custom ROMs?
There is only one custom ROM for this device, the Lineage OS 14 but I advise against using it because it has a bit of a bug but I haven't had more time to work on it yet.

D) But wasn't the guide different?
It has been updated with a better method with more advantages.

E) My device is in Hard Brick! How can I solve?
There are various types of Hard Brick and it depends from situation to situation.

F) I have installed Lineage OS 14 but I am not satisfied, I want to go back to Flyme!
Flash update.zip from official website.

G) Can I relock the bootloader?
From MTKClient readme:

                                                                        for relocking use:
python mtk da seccfg lock               
                                       

H) TWRP? I am interested in Device Tree, where is the link?
Here: https://github.com/XRedCubeX/twrp_meizu_m5c

I) I have a Meizu A5 (Chinese M5c) will it work on it?
I can't assure you anything but it could work, the only way to be sure is to test on a Meizu A5.

J) Can I flash twrp from fastboot?
Fastboot refuses to flash partitions, strangely, I don't know if downgrade or anything changes the situation but for now I was able to flash it with mtkclient.