FWIW, I have the full subscription to North America databases...nav, obs and terrain. IIRC, it's Worldwide Terrain data, and it almost *never* changes (think about it ). I think the last update was a couple of years ago.
ETA: left card is nav, right card is terrain/obstacle data.
Unfortunately Terrain data does change. A newly built antenna can ruin your day.
You may want to make sure to understand the difference between the terrain database and the obstacle database.
My G430 (non WAAS) doesn’t have a terrain card (slot but no card), just a NAV data card in the other slot. For terrain/obstacle data I use the GRT navigation data displayed on the EFIS units. The 28 day data updates cover changes to obstacles that have changed such as windmills and towers, etc.
It's true that you could use an EFIS database for terrain and obstacles instead of something like a GNS-XXX or whatever. I just like complete datasets for everything (kinda OCD, I guess).
But I'll bet that the obstacle database even on an EFIS like GRT, Dynon, etc., is really just repackaged FAA obstacle data, which is released every 56 days. Still, you are correct...you don't need both the EFIS and the GNS to have the data.
...but charge for their system updates.