I use the OH - synology package (I currently use the 2.5.2.000 version) only to have the start/stop mechanism from the package manager. I manually download and unpack newer versions of OH and make a link openHAB to the current version.
lrwxrwxrwx 1 root root 28 Feb 13 21:22 openHAB -> openhab-3.1.0-SNAPSHOT-2206/
drwxr-xr-x 7 openhab users 4096 Aug 19 11:01 openhab-2.5.5
drwxr-xr-x 7 openhab users 4096 Oct 15 21:05 openhab-2.5.7
drwxr-xr-x 7 openhab users 4096 Dec 19 18:32 openhab-2.5.9
drwxr-xr-x 6 openhab users 4096 Dec 23 21:47 openhab-3.0.0
drwxr-xr-x 7 openhab users 4096 Dec 22 21:17 openhab-3.0.0.RC1
drwxr-xr-x 6 openhab users 4096 Jan 15 20:20 openhab-3.1.0-SNAPSHOT-2144
drwxr-xr-x 6 openhab users 4096 Feb 3 19:41 openhab-3.1.0-SNAPSHOT-2177
drwxr-xr-x 6 openhab users 4096 Feb 13 21:20 openhab-3.1.0-SNAPSHOT-2206
Of course there are some additional things to do because the original SPK version have some directory linked to the home directory of the user openhab. That needs to be done manually.
In addition you must copy the ui directory from the original installation from the spk (openHAB/ui) to your new openhab directory, otherwise the package manager does not detect a running OH. I have never tried to figure out why.
And of course the above mentioned manual install of java11 and adoption of start.sh is needed.
Br,
Stefan