It requires UNIX knowledge you seem to don’t have (keywords chmod, vi etc).
Unfortunately some of this is even specific to Synology Linux which all by itself already isn’t really standard UNIX. Plus Synology openHAB packages aren’t up to date.
So as a well-meant advice: consider moving away from Synology before you invest too much time.
Go for a RPi with openHABian instead.
They’re similar and Synologys suffer from the same problems as do RPis. A RPi is even faster in fact.
OH on a RPi IS well fast enough as long as you pay attention to a couple of things.
Most important, you mustn’t interactively edit items and rules files (unless you’ve completely understood what’s going on then).
Startup also takes long (but once running there’s no need for a restart so simply ignore that).
Both of these are actually OH problems not RPi ones. They just are more visible on RPi than on faster machines.