Very, very little maintenance. If there's an update for something on docker, a simple click in the container manager, and it's done. Yes, I can automate, but prefer to manually do these as many of the docker apps I use are in high development and I like to know what's changing with each version.
Synology packages update easily, and the system updates happen only once in a while. A click and reboot.
I've tried to minimize things as much as possible, and to make things easier for me. One day, someone in my family will need to take over, and I don't want to over-complicate things for them, lest they lose all our family photos, documents, etc.
I probably spend more time keeping the fans on my actual NAS clean of dust, than I do maintain the software end of things. LOL