How to safely downgrade P30 Pro? What we need to pay attention when selecting firmware?

if it runs android 9 then you can downgrade only to android 9 lower version.

Failed to flash Mate 9 MHA-L29

DC Phoenix 0.0.0.117
user:039C22261C

Looking for a device in force upgrade mode
Device detected:
COM48: HUAWEI USB COM 1.0 (COM48)
04/11/2019 10:25:46 PM Starting to write device in FASTBOOT mode...
File to update: C:\Users\Arslan IT Center\Desktop\Huawei Mate 9 HLMHAAL00AM00A074 Board Software Android 7.0.0 Nougat EMUI 5.0 05021YVS File By (Suppor.**************)\HLMHAAL00AM00A069_Board Software_China_Nonspecific_Android 7.0_EMUI 5.0_05021YVS_A_2._inc_testpoint.dgtks
Device detected:
COM48: HUAWEI USB COM 1.0 (COM48)

Process identifier:20121494


Writing bootloader...
Step 1...

USB Device REMOVAL
Type: @oem213.inf,%qcomdevice00%;HUAWEI USB COM 1.0
VidPid: VID_12D1&PID_3609
Instance id: 5&22a600f3&0&9


USB Device INSERTION
Type: @oem213.inf,%qcomdevice00%;HUAWEI USB COM 1.0
VidPid: VID_12D1&PID_3609
Instance id: 5&22a600f3&0&9


Error writing Bootloader
04/11/2019 10:26:00 PM Writing device finished - INCOMPLETE

Doesn't seem that loader fits, should try another one from the list on oeminfo tab.