Deco schedule only adding time to a 6m ceiling

You're asking more of general Bühlmann ZHL-16c deco algorithm question than a Descent product question. If you're only going to 30m then it's not surprising that the the first mandatory deco stop is only at 6m. Try planning a longer dive to a depth of 65m and you'll see the first stop depth move down to like 24m. If you really want deeper deco stops then you can configure a lower GF Low, however the latest research indicates that you probably want a value closer to 70 than 50.

I am using a Japanese version, so terminology may differ, but go to dive setting by pressing and holding a left middle button, then scroll down to deco stop.  You can choose the final deco stop ceiling depth from 3m, 4.5m or 6m.  3m is the default setting.

I don't go that deep, so I don't have the answer, but changing the setting above may help the situation.  Or maybe 6m is the deepest deco stop depth by Garming MK3.

No, that's something different. Garmin dive computers implement the Bühlmann ZHL-16c deco algorithm and can generate much deeper deco stop depths than 6m. I think the deepest deco stop I have personally seen was about 24m but others have certainly seen more.

The configuration setting that you're referring to is the shallowest stop depth. Instead of doing stops at both 6m and then 3m, some ocean tech divers prefer to roll the 3m stop time into the 6m stop. This is slightly less time efficient from a deco time and gas perspective: offgassing is slower at 6m than at 3m. But it's a lot harder to maintain buoyancy control at 3m, especially when doing a drifting ascent under a marker buoy in big swells. So, we just stay down at 6m until we clear our deco obligation, then make a very slow ascent to the surface without doing a real deco stop at 3m.