Why is my Mac mini 2011 — with an SSD upgrade — stuck on grey screen during startup?

A colleague of mine gave me an old Mac mini (Mid 2011 2.5GHz, model A1347 ) that I wanted to put back to work mainly for fun. The Mini came without an HDD, so I bought a new SSD (Samsung 870 Evo, 500GB) and installed it inside the Mac following the guide at iFixIt with no problems. Then, I created a bootable USB flash drive with macOS High Sierra (10.13), known to be compatible with my model. However, when the loading bar below the Apple logo arrives to about 70%, my monitor seems to reset and a grey screen appears, and nothing else happens. I tried to leave it for some hours but nothing changed. I also tried to boot from internet recovery: It successfully downloads the internet recovery image, but then after the loading bar arrives ~70%, it gets stuck on the same grey screen. I also launched Apple Diagnostic (holding D and later Option+D during startup), run the "long" test and it says *no troubles found*. I also tried the following: - Reset the NVRAM and SMC. - Boot with cmd+option+R, cmd+shift+option+R, and cmd+R (both with Wi-Fi and Ethernet) - Boot with cmd+S to get into single user mode: here everything seems ok, I run fsck on every /dev/disk* and it tells everything is fine. Then I type exit to continue the boot process, and it gets stuck to the same grey screen. - Boot with cmd+V (i.e. verbose mode), there don't appear to be errors where it gets stuck, instead, the screen again blinks and goes gray forever (same as above). - Tried to use the bootable USB stick with other OS versions, such as Yosemite (10.10), El Capitan (10.11), and Sierra (10.12). - Connected an HDMI display instead of DisplayPort from the Thunderbolt port. None of these worked, the Mac always gets stuck to the grey screen. I do not think it has a discrete GPU, and even if it had it, I tried some boot-args to disable it with no success. Is there anything else I can try to get this machine back to work?