Android won't start and need help flashing

Hi,

I have bought a used HTC M9 on Kijiji (I'm in Québec, Canada)
It must be faulty because it kept randomly restarting (sometimes once, sometimes for periods of 20 minutes, which made it hardly usable)

After clearing the cache partition and factory resetting it a couple times, I decided to flash it.

What I did:
- enabled the developer mode and USB debugging
- unlocked the bootloader
- flashed TWRP
- flashed SuperSU v2.74-2

Now, Android won't boot. When I start the phone, I see the green HTC logo on a white background with a red text saying it's a development build, then after about 10 seconds it restarts to the same screen. Over and over again.

In download mode, I am able to see the device with fastboot, but adb does not show anything.
In recovery mode, it's the opposite.

The phone is still S-ON so I have tried S-OFF methods (moonshine and firewater), but the fact of not having fastboot AND adb working at the same time made them fail.


Here's my phone info:

(bootloader) kernel: lk
(bootloader) product: htc_himaulatt
(bootloader) version: 1.0
(bootloader) imei: ************
(bootloader) version-main: 3.38.666.2
(bootloader) boot-mode: download
(bootloader) version-baseband: 01.01_U11440801_96.01.51207G_F
(bootloader) version-bootloader: 1.0.0.0000
(bootloader) mid: 0PJA11000
(bootloader) cid: BM___001


Any help would be greatly appreciated...
Thanks in advance!