[Redmi K20 Pro / Xiaomi Mi 9T Pro Guides, News, & D] Talix:
Thanks for all your comments guys, many of them helped me in my journey to fix my wife's hard-bricked device... I did it without paid help, without MiFlash (although I tried many versions, none worked - all resulted in EDL auth related errors) and without the OP's paid method. IT IS NOT NECESSARY.

For everyone's benefit I'll post my experience here. Please note, this was for my own records as well, so it's not exactly well written. some of the expressions are made in a way that helps me remember what I did and the reasons therefore. The links may also expire at some point, but I've written it detailed so that the updated resources may be possible to find even then. The instructions may only make sense to those who have done some research themselves... :p

My wife had her phone, Xiaomi Redmi Note 6 Pro, Global, hard bricked after it died of low power... We never flashed any custom rom or anything and the Bootloader was locked. Completely stock phone, hard bricked for no apparent reason.

It reported as device "Qualcomm HS-USB qdloader 9008" in device manager when connected via USB, signifying broken BootROM. Only EDL flashing will work in this instance. Recovery and fastboot mode are all unavailable. The battery doesn't charge when plugged in and the LED was completely unresponsive too.

After much research I found the following fix (https://xdaforums.com/redmi-note-5-pro/how-to/fix-arb-bricked-device-qfil-t3841780) on the "Redmi note 5 Pro" forum. Below I've abbreviated and commented the procedure with what I found helpful:

1) Download QPST/QFIL (most recent version was 2.7.474) - MiFlash won't work without an EDL enabled/authentication account usually reserved for Xiaomi service centres.

2) Download fastboot rom suitable for the device (https://c.mi.com/thread-2265065-1-0.html) Mine was Redmi Note 6 Pro Global V10.3.2.0, 2019-04-26. This has to be of the recent version at least the version that was installed when the phone bricked or newer, else it could hard brick again. It also has to fit the version (China/Global/RU) or it may get stuck in recovery mode. Unpack the ROM until its a folder in explorer (TGZ -> TAR -> folder). Put it in root of C-drive.

Another link for ROMs, seems more updated: https://mirom.ezbox.idv.tw/en/

However, make sure to get the FastBoot rom, NOT the Recovery or OTA firmware.

3) Download patched prog_emmc_firehose_Sdm660_ddr.mbn and copy inside fastboot rom images folder. (http://www.mediafire.com/file/924uilw13kx9ujr/prog_emmc_firehose_Sdm660_ddr.mbn/file)

4) Take of back cover. Unplug battery. Short the two "test points" while plugging in the usb cable - This will boot in EDL flash mode. (Google where to find the specific one, I used this link: https://www.mobilerdx.com/2019/03/mi-redmi-note-6-pro-test-point-redmi-note-6-pro-edl-mode.html - Had to remove back cover, two metal shields and the touch connector to get to test points and remove battery... On most phones it's only necessary to remove back panel)

5) Start up QFIL.exe (in bin folder in QPST install directory). It should detect the port the phone is connected to.

6) Follow this video for how to configure the flash: https://www.youtube.com/watch?v=H2z9GRXNhKE

7) Flash it... It may not work the first time. Redo the USB step while holding the test points and try again.

Also, if there are issues, possibly troubleshoot using MiFlash. The error codes may give a clue to what is wrong. The errors I've experienced was:

- Cannot receive Hello message - Signifies that flash mode is not active - Restart holding test points. I've also seen forum posts reporting this can be caused by the computer's USB port (try another) or the windows computer having "driver signature enforcement" enabled. Google how to disable if needed. I did disable it (restart method) but I don't know if it made any difference as I still had the error and then proceeded to use the test point method, which worked. It could possibly have contributed to my success.

- Not authorised (or some equivalent) and "Only Nop and Sig Tags allowed before auth" Depending on the MiFlash version you can get some variant letting you know that EDL authentication is required. This is only a limitation if using MiFlash. QFIL will do the job no worries. No need for authentication.

8) When it's finished flashing, unplug USB and reset the connectors for battery and touch panel. Don't worry about the fingerprint sensor on the back cover, it's not needed for initial setup. Then plug in the USB to allow the phone to charge. Turn on the phone holding the power button for 5-10 seconds, until it vibrates.

The phone may now start up and seem stuck on the Mi logo... Mine was sitting like that for about 5 minutes before the little "loading" indicator dots appeared as well on the bottom of the screen. After about 10 minutes in total I had the start screen! YEEEEESSS!!!! After activating the phone authenticating with my wifes Mi account it was all good.

Hope someone can benefit from my experience :)