Thanks a lot for your suggestions! Some really good points have been made, which have helped me to re-assess my calendar situation.
@ScottL: I found the thread you mentioned -- thanks for pointing it out.
The slightly intimidating habit of "duplicate writing" that comes with using 2 calendars, one digital and the other paper-based, is what I see as the main Achilles’ heel in maintaining such a system.
On the one hand I like Google Calendar because it has a nice clean layout and it's just fun to use. Yet on the flip-side, and perhaps more importantly, using a paper-based calendar gives me the advantage of having it with me at all times, and I also must admit that paper does feels more natural to me. When it comes to calendaring, pen and paper works best for me I think.
So here's my current solution:
I started using Google Calendar to print out each month of the year. Then I took each monthly printout (May, June, and July for now) and fitted these inside my main 3-ring paper planner, so that each monthly foldout appears at the beginning of each tabbed month in my planner. So if I flip over to the June section, I now have a nice month-at-a-glance view for June, which I can refer back to and update on the fly at any time. This kind of system is working well for me so far!