Mac mini with Sierra loops at startup

Hi,

As subject says, I have a mac mini (5.2, mid-2011) with Sierra (10.12.5) that simply loops at startup. I've been trying quite a few things with no success. To recap:

- normal startup, it loops

- Safe mode / Recovery Mode: it hangs

- Internet Recovery mode : it works (meaning, it connects and start booting) then it hangs.

- External USB: I prepared a bootable USB with High Sierra, it let me choose the external device, it starts (meaning, apple logo with bar), then it hangs at 80%

- Single-user mode: I get in no probs

- fsck -> "appears to be" OK

- hardware check comes back OK

- disk has all the content (I was able to enter in Target mode and save everything, just in case ... ) (disk is 93% full)

- Verbose: I get a bunch (literally, hundreds) of errors for kext (invalid signature). Before rebooting I get

- AirPort GetSSIData: Get Failure

- Dynamic OEM (seems OK to me)

- unexpected session: 100000 uid: -1 requested by 52

- AppleKeyStore: operation failed (pid:52 <etc etc>

- IOConsoleUsers: time(0) )->0, lin 0 llk

- gIOScreenLockState 3, hs 0, bs 0, sm 0x0

<reboot>

clearly, I reset the NVRAM with no success.

If anyone has any idea ... thanks!