Hi again, I just tried the C172 to understand the problem, and thanks to you I discovered the vertical navigation section on the "G530". Nice!
This vertical navigation page helps you manage your top of descent and helps you adjust your vertical speed if you set it right, in order to make your chosen constraint. I think you understood how to use it.
But you have to manually engage VS mode and manually adjust the vertical speed. Once you reach your altitude, you will have to press ALT on the autopilot, otherwise your Autopilot will remain in vertical speed mode.
Now, if you want to fly manually for altitude and automatically for lateral guidance (ie NAV only), I thought you just had to press ALT again so that the autopilot is in NAV mode only. Somehow it didn't work, the autopilot still wanted to remain at the previous target altitude. So the only option seems to be to deactivate the autopilot by pressing AP and making sure the digital screen is blank. Then you have full manual control, which is the proper way to do when there is no ILS.