Mac Mini 2018 model A1993 MLB 820-00939-A - does not boot, shuts down

[Vogelek23]

Hello. The Mac Mini 2018 starts up but shuts down just before showing the OS logo and progress bar, then restarts in the same way in an infinite loop. When I do not touch the keyboard, Mac Mini starts up (white LED lits up) and shuts down after 4-5 seconds with no picture on HDMI screen (but screen is initialised as it goes to energy saving when Mac shuts down). Then it starts up again in the same way in an infinite loop. When I hold eg. Alt key, the logo appears followed by the boot screen, and I can select the startup disk but neither the internal nor external disks can be started up from as of an immediate shutdown. What I did with no luck (still shuts down and restarts):

1. Replaced RTC battery (old was 2.8V but replaced just in case)
2. Replaced RAM memory (100% tested)
3. Started Mac up with a single module in each slot
4. Tried to boot to Recovery mode
5. Tried to boot to ASD
6. Tried to boot to Safe Mode
7. Revived T2 chip in DFU mode
8. Resoldered CPU (full reflow with all balls melted)

All the voltages are present, it is just a sudden shutdown right before the loading logo and progress bar appeared. Where should I focus my attention to?

 

[2informaticos]

Did you try DFU restore?

 

[Vogelek23]

Unfortunately, I cannot restore BridgeOS as of the very high importance of data. I will now try Target Mode and OS backup onto the external drive.

 

[Vogelek23]

I can confirm that Target Mode did the trick regarding data retrieval, so I can now fully restore the BridgeOS to see if it helps to rectify the main problem.