How can I build a long-lasting composite deck that’s exactly flush with the ground?
Use the composite planks as supports.

Create a void where you would have your deck.  Make it two plank thicknesses deep.  If your ground is soggy you might want to go deeper and level up with chip-to-dust and a vibrator.

lay a weed suppressant membrane.

lay your supports.  These are the same as your boards but spaced out  at 14" centers.  You might want to drive 6" nails into these at either end to locate them horizontally.

Lay you boards over the top with deck screws.  Job's a good'un.