Happy new year indeed. I finally unbricked my phone after checking other XDA threads related to softbrick. I found my answer at this thread.
https://forum.xda-developers.com/mate-20-pro/help/probably-soft-brick-update-to-emui-10-t4003915
We all know that the reason for softbrick is that the firmware does not match the phone original region/vendor/country. For many of you, you need to second-guess the true region/vendor/country of the phone.
But from this post, it seems that only the base file is installed (EMUI 10), while the cust and preload files failed to install (remains EMUI 9.1), as this post suggested:
https://forum.xda-developers.com/showpost.php?p=81258527&postcount=10
And after reading further, it seems the simple fix is to rename the cust and preload file to match the phone region:
https://forum.xda-developers.com/showpost.php?p=81098065&postcount=89
The next question is: what is your phone's region/vendor/country? Instead of trial-and-error, I use fastboot to check my phone. My result:
c:\adb>fastboot getvar vendorcountry
vendorcountry: normal/hw
So, what I did was to copy the 3 dload files to the sd card formatted to exFAT, and rename the 2 files.
From
update_sd_cust_VOG-L29_xx_yyyyyy
update_sd_preload_VOG-L29_xx_yyyyyy_R1
To
update_sd_cust_VOG-L29_
hw_normal
update_sd_preload_VOG-L29_
hw_normal_R1
I was pleasantly surprised that it works. But in my case, there is this notification on the dropdown that says I am using an unofficial version. The firmware I used to install is L29 10.0.0.173 (C431E19).
What's next is that according to
@oslo83 the cust and preload need to be corrected, but I'm not sure what if I don't correct it. Guess I will only find out next year.