[Sony Xperia M4 Aqua] Mimi1001:
I think my device is pretty much done for, I suspect some hardware related issue, but maybe some of you wonderful people here (if there still is anyone here) could still help me.

So somehow my M4 Aqua suddenly bricked without my interference, I had been using it regularly in the morning and at noon time, when I wanted to check on it, it simply wouldn't turn on or display anything at all. Even after pressing all the buttons for a considerable amount of time, no reaction occurred, the screen remained black. When I forcibly shut it down using the button next to the SD card slot, I seem to could get it on by using the power button, but the device doesn't seem to boot at all. All it displays is the Sony Logo, no bootloop, just the Sony Logo even for hours, presumably until the battery runs out.

Yes, my phone was rooted (using files from maybe this or this guide, can't really remember) even though I haven't been taking advantage of it, all I basically did was automatically denying SU permission for Link2SD which I had installed. I also should have the custom TWRP recovery, but I cannot access it, it leads to the infinite Sony Logo every time. As I initially rooted the device, I had to flash a new stock firmware which might not be suited for my device. (I honestly had no clue what my exact model was, if it was a E2303 or E2306 or whatever, but didn't bother much since the new firmware worked just fine for months)

Even when I simply want to charge the device, the generic battery symbol won't appear anymore, instead being also replaced by the aforementioned infinite Sony Logo. If I unplug the USB, the red LED light and the Logo persists which lead me to think that the device somehow freezes mere seconds after startup.

I have also noticed that the M4 Aqua takes suspiciously long to force shut down, I have to press the little button for ~15 seconds for the screen to go black, when it really should be about 5. I have thus removed the back case to fasten the process by removing the battery directly. (Did that break something?)

I can connect my phone in fastboot mode to the PC, but it seems to freeze yet again. I can perform various actions in Flashtool, but it doesn't seem to affect the M4 Aqua, when I try to restart the phone into system for instance, Flashtool doesn't report any errors, but nothing happens to my phone. Just how the red LED persists, the blue light does too when disconnecting from the PC until I forcibly shut down the Xperia.

Flashing a new firmware is not possible as neither Flashtool nor Xperia Companion recognize the device in flashmode even if the green LED clearly lights up. Like the other LEDs, this one also persists after disconnecting from the PC. I have been using Flashtool Version 0.9.32.0 first and then downgraded to 0.9.23.1 since that one came with the firmware I had installed, but to no use.

All this leads me to suspect a hardware issue, likely with the RAM (both sticks failing simultaneously) as I gathered from other threads and posts on the internet that that is a problem with the M4 Aqua. The CPU overheating is also an often cited problem, but I think I can rule that out. I have tried to cover the back with my hand absorbing the heat, slightly bending the phone and even put it in the freezer, but nothing changed at all. I have removed the SD and SIM Card to check if they were faulty too.