Only 200kg? That's good. Not much at all for an imposed load.
The last time I did a deck very similar to this, it was a steel structure designed by a Structural Engineer, and made up by a local Steel Fabricator.
All pretty easy, just bolt everything together.
Since the imposed load is so low, I would approach a Fabricator and ask them to draw something up. It hardly needs an Engineer.
The steel sections would be small enough for you to handle and assemble.
It would be simply a square/rectangular frame with crossmembers .
On top, you could then set ordinary timber joists, which would carry the timber deck in the usual way.
The frame would be carried on "legs". Very short ones at the top, longer ones at halfway (2metres), then the longest at the furthest point (4 metres.)
Usually called "stanchions", these legs would be bolted to a concrete pad maybe 300mm x 300mm. These pads would extend down into the ground rather like a normal concrete foundation.
Retros's "ground screws" are better suited to level ground in my opinion. Here, you need something that will resist forces trying to make the whole structure slide down the hill ;o)
Foundation depth depends on the ground quality. Usually one metre would do. With hard ground, maybe a bit less.
Oh, and the steels would need to be galvanised. No problem for the fabricator to arrange that before he delivers to you.
Or, as you say, you could make the whole thing yourself, in timber.
If you would prefer timber, let me know the approx. overall length of the deck. I know it goes out 4 metres, but I need the length.
Have a think ;o)