During investigation of "what to do next?" on Xpenology Community a user ilovepancakes suggested running VDSM instead of fighting with DS-intended DSM distribution. Here's (somewhat scatter-brain) documentation of the findings.
These texts were copied mostly 1:1 from my forum posts in the topic linked above. As they were made as I was discovering things some details may be corrected in the later part of the text. After getting to the licensing part I decided to take a different approach - a franken DSM ;)
- VMM: Virtual Machine Manager; a QEmu-based hypervisor + GUI present in the DSM
- VDSM: Virtual-DSM; a special distribution of DSM which can officially be run under VMM
- PAT: an archive format used by Synology to distribute system packages
- VDSM exploration: getting sense of how it work and attempting to use it outside the VMM
- Franken DSM: attempting to pair a DS (e.g. 3615xs) OS with VDSM kernel