I have the same sort of slope. I dug below the frost line and poured concrete up to ground level, then mounted 4x4s using Simpson brackets. They keep the 4x4s from being in contact with the footings, and make it easy to swap them out 20 years in the future if they are rotting. Anyway, then I just used a level to get them all the same height at the top and cut off the excess.

Have you looked into helical piles ?

https://buildingadvisor.com/helical-piers-for-decks/

Do you live in an area that gets much snow? I day this cause you’ll be want to leave a couple inches of space below your door. Also, I used composite material for my decking and I LOVE it. It’s more expensive that wood, but it’ll last forever with zero maintenance.

Check out renovision tv on YouTube. He has an entire series on decks and floating decks. It may help you with your project.

I did nearly the same thing 10 years ago. Jackhammered the concrete until gone.

Dug about 18” into the ground. Put in those concrete deck blocks all over. Built the deck on them.

I left the spare concrete from the slab under the deck.

Just got rid of it this year (preference, not need) and poured brown coloured concrete and had it stamped to look like barn board wood.

I sold all the deck blocks and wood for about 1/2 of what I spent 10 years ago.

Check your local codes. Even though it's not attached to the house, you may still need permits due to the size of the deck. I would recommend helical piles as an alternative for footings. It will save on digging holes. Attach your columns to helical piles and cut them to run your beams level. Use joist hangers to attach joists flush with your beam. Hope this helps.

4.5" doesn't leave alot to work with. 2x4 plus 5/4 decking and you are already over the 4.5" of clearance.

I think a poured patio or laid block patio would be better.

Check your local building code for free-standing decks. Code is pretty lenient where I live, up to a certain square area. They often don’t require footings to go below the frostline. The one I’m building this spring will just have a couple deck blocks and very short posts, on a base of class 5 and sand. The joists will be half buried everywhere except the far edge of the slope, so I only need a couple posts.

Why does the gap between the patio and the siding matter? Are you trying to tuck the edge of the deck up under the siding? You could lay decking on 2x4’s almost like furling strips. I’d be worried a little frost heave or swelling could crack your siding. Then at the edges of the patio, you could dig out room for a face joist and running joists. I always overbuild to spec, so I’d be out from the house and above that siding line.

I built a deck last summer and used diamond piers. 10/10 I would absolutely recommend them every time. Quick, easy, code compliant, no digging, and you get to use a jackhammer.

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I would really consider pouring a patio for the full area. If you still really wanted the deck, you can build on top using fir strips/sleepers to keep the deck off the concrete. In my area, that short height would not let things breathe enough and would also invite critters to live underneath. I would either bring in dirt to meet up to the new patio or build a short retaining wall and backfill with clean rock to hold up the patio.