[Android Q&A, Help & Troubleshooting] :
This was wonderful, thank you! I've added some of my own notes below as an experience of what I encountered when attempting this process myself (spoiler'd because it is a lot).

I do not have enough post count to add links, but titles to the relevant articles has been added.

Follow steps in Article "Guide: How to Unlock Xiaomi Qin 2 (Pro) and Install Custom ROMs" from step 1 to (and including) step 10 (Unlocking section).

Notes:

- A Linux PC is necessary.

- You'll have to mark the "fastboot" file from the "Android_device_unlock.rar" archive as executable (chmod +x).

- Run the "fastboot" file as root.

- Getting the "SPD Research Tool" to pick up the tablet and not let the tablet try to move to either the charging

screen or the bootlogo is difficult, but do-able. Press and hold Power+Vol_Up and release when Windows does its

USB device detected chime.

- Flashing takes a few minutes (I think around 300 seconds).

- The SPD Research Tool extracts the PAC file contents into a folder. Grab the system images from there.

- The "avbtool" is available to be cloned via git from Google's repo

- The avbtool is a python script that is patched with three lines of code at line 1776:

kfd = open(self.partition_name, "w");
kfd.write(self.public_key);
kfd.close();

- When you use the patched avbtool on the vbmeta-sign.img file you copied (avbtool info_image --image vbmeta-sign.img)

it will produce several partitions with relative public keys that need to be stored in separate files for the next step.

The contents of the files are simply the public key and the partition name as the file name. Store the files in a folder named "keys".

- When creating the makefile, ensure that proper indentation is used. The code segment below is properly formatted (hopefully). If you get make errors, remove and re-indent the lines.

- If your "rsa4096_vbmeta.pem" keyfile is not placed alongside the makefile, ensure the --key flag points to this file.

- The makefile exists in the same directory as the system images.

- I had to insert local paths to the avbtool, as it was not installed to the system PATH.

- The dhtbsign-vbmeta.py command is located below. Make sure to mark this as executable as well.

Everything else is rather straightforward.

# makefile

all: boot-sign.img recovery-sign.img vbmeta-sign.img

vbmeta-sign.img: makefile avb4096_pkmd.bin keys/*
avbtool make_vbmeta_image --output vbmeta.img --padding_size 16384 \
--key rsa4096_vbmeta.pem --algorithm SHA256_RSA4096 --flag 0 \
--chain_partition boot:1:avb4096_pkmd.bin \
--chain_partition system:3:keys/system \
--chain_partition vendor:4:keys/vendor \
--chain_partition product:10:keys/product \
--chain_partition dtbo:9:keys/dtbo \
--chain_partition recovery:2:avb4096_pkmd.bin \
--chain_partition l_modem:5:keys/l_modem \
--chain_partition l_ldsp:6:keys/l_ldsp \
--chain_partition l_gdsp:7:keys/l_gdsp \
--chain_partition pm_sys:8:keys/pm_sys \
--chain_partition dtb:11:keys/dtb
./dhtbsign-vbmeta.py vbmeta.img vbmeta-sign.img
@rm -f vbmeta.img

avb4096_pkmd.bin: avb4096.pem
avbtool extract_public_key --key avb4096.pem --output avb4096_pkmd.bin

avb4096.pem:
openssl genrsa -out avb4096.pem 4096

boot-sign.img: boot.img avb4096.pem
cp boot.img boot-sign.img
avbtool add_hash_footer --image boot-sign.img \
--partition_name boot --partition_size 36700160 \
--key avb4096.pem --algorithm SHA256_RSA4096

recovery-sign.img: recovery.img avb4096.pem
cp recovery.img recovery-sign.img

# dhtbsign-vbmeta.py file (from "How I Unlocked Xiaomi Qin 2 Pro and Installed Phh GSI")

#!/usr/bin/env python
import hashlib
import sys

f = open(sys.argv[1], "rb")

b = f.read()

sha = hashlib.sha256(b).digest()

f.close()
f = open("vbmeta_signed.img", "wb")
f.write(b)

f.seek(1048576 - 512)

f.write(b'\x44\x48\x54\x42\x01\x00\x00\x00')
f.write(sha)
f.write(b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x40\x00\x00')
f.seek(1048576 - 1)
f.write(b'\x00')
f.close()