DISCLAIMER: I'm not responsible for anything happens with your printer if you decide to follow steps in this posts.
Preparation
To have Anet A8 v2 running Marlin 2.1.2 you will need ST-Link V2 to flash the MCU. It looks like this:
Get this first. Take a note on the labels, the one you got might not have the same label as the picture. It is important because you will have to match them to the mainboard.
Go to Marlin website and follow the instructions from there to install softwares needed. I use this variation. Please read the instructions carefully. As an addition I also use Auto Build Marlin.
Modifying Marlin
There are some files to be added/modified, I won't put the details here, you can see it on github. The required files are:
[ini\stm32f4.ini]
[Marlin\src\pins\stm32f4\pins_ANET_A8_V2.h
[Marlin\src\pins\pins.h]
[Marlin\src\core\boards.h]
[Marlin\Configuration_adv.h]
[Marlin\Configuration.h]
After adding and/or modifying the files according your printer setup, you can build it and flash it.
Flashing A8v2
Connect ST-Link v2 to the mainboard, check the label first. Pins must be connected to the correct destination or you may DAMAGE your mainboard. If you're not sure, my advise is don't do it at all.
You only need to connect 3 pins: SWDIO, GND and SWCLK. Mine is pin 2, 4 and 6. DO NOT connect other pins especially the ones with voltage (Pin 7, 8, 9, 10), unless you're know what you're doing, just don't.
Connect the pin G to pin GND on ST-Link V2, pin C to SWDCLK and pin D to SWDIO.
After connecting it, check it again, and check it again. Just make sure it's the right pin connected to th right pin.
Then connect ST-Link V2 to your computer and upload the compiled Marlin firmware with VSCode.
If you're successfully upload the firmware, you will see the printer rebooting and showing new screen.
And congratulation, you've made it.
[Edit: 2023/05/06] I'm removing my Marlin fork. The reasons are, first of all, I think it's better for Anet A8v2's users to open feature request to Marlin repo. That way it's officially supported. Hopefully Anet will give Marlin Dev's their source codes. The second reason is my codes are experimental and tested only with my one and only printer, even though I've been using it since I made it, I don't think it's for average users who expect things to "just works".
If you still want the codes, I'll be posting the codes on my shop here: https://www.buymeacoffee.com/yonitjio/e/134843. It'll cost a little, but from my side, if you're willing to spend some money, you're should already research on it. Although, the condition will be the same, it's an experimental and it's only modification the codes, I will not provide the whole Marlin code nor the binary files, or support or anything else.