I know it doesn't make sense, but it is how it is.
Installation: baremetal on Intel Celeron G1820 on an Asrock H81M-DG4 motherboard with 4 GB DDR3 RAM. Two Seagate HDDs, 160GB and one IronWolf Pro 4TB.
I flashed Jun's v1.03b Loader for DS3615xs on a 4 GB Kingston DataTraveler USB stick using Balena Etcher, but I could not find the DiskStation at all in the network. Jun's 1.04b Loader for DS918+ worked and the DiskStation was found, but I got an error which was solved by changing SataPortMap to 2 in the grub config.
I proceeded to install DSM 6.2.3-25426 for DS918+ (don't know why I did not go straight for Update 3). Got an error towards the end of the installation: Installation ERROR - The file is probably corrupt. (13). I double checked the PID and VID of the USB stick and there was no issue there; but I used a grub prompt to add them again, along with other commands:
vid 0x0951
pid 0x1666
append SataPortMap=2
boot
and the error went away. The installation succeeded, but when I tried to reboot, the web interface would not come up. The only thing shown on a locally connected monitor would be a blinking cursor.
But inputting the commands again resulted in a successful boot. Through elimination, I found out the first three commands are not actually needed, but the boot command is absolutely required.
So now, in order to be able to successfully boot, I press the c key and type in boot after every restart. Everything else works smoothly, this is the only thing nagging me.