So, I Googled the Test Point method and it worked like a charm!
Thanks for the info!!
Basically you open the phone's back, you unscrew a small metal plate and then you have access to 2 tiny dots, the "test points", which apparently are built exactly for such purposes.
You unplug the battery (and the display I think although once it worked with the display plugged so don't take my word for it), temporarily connect the test points with a pair of tweezers or a paperclip 📎, while the test points are connected you plug the phone to the PC and it miraculously shows up in device manager under "ports" as "Qualcomm 9008"! (Qualcomm drivers need to be installed. If it shows up as "diagnostics..." anything, you did something wrong).
The device is flashable in that state, you use XiaoMiFlash to flash a stock ROM (it's a folder that contains a folder named "images". The flash tool doesn't have you selecting specific files to flash, you need to select the whole images folder).
Left: the screws you gotta unscrew
Right: the test points and what you need to unplug
PS on opening the back: insert a flat narrow tool (best to have a smartphone repair kit) where the back plate connects with the front part of the phone (NOT where the glass meets the rest of the device). At first it won't go in but be persistent and patient, eventually it it. Wobble it back and forth and go slowly around the whole device the same way, paying attention not to wound the parts under the side buttons. Once you do that, it'll be possible to remove the back with your hands. Don't yank it because you need to carefully unplug the tape connecting the fingerprint sensor to the motherboard first