I couldn't find anything that looked suspicious on my system, so I did the following:
Moved every folder from /var/packages/ to a temporary directory.
I didn't know of any other way to actually disable all the services in the DSM.
After that the unit rebooted fine.
So I then manually moved the services back one by one, and rebooted each time.
My problem was with the debian-chroot package from Synocommunity.
Apparently when I added a debian service to be initialized along with the system, it caused DSM to hang.
Maybe I have to end the startup command with &.