What do you plan to use the hill/bottom of hill for? I think adding dirt doesn't sound like the right answer, but if it would be to create a switch back as mentioned then it could be. It really comes down to what your usage would be. Or did you mean add dirt to level out the yard and then a retaining wall on the perimeter?

Yes...the deck still has round hole / cover where hot tub used to be. I have purchased treated wood to make the deck "whole" again, covering the spot. I find it hard to believe that one has to go through this process every couple of years, even having sealed it. ...Wonder if there is anything hard-core that will preserve longer. I was told paint is even worse and that Trek comes in last. Thanks for all your input.

The pros will want more pictures as well as dimensions to give you advice! Also Houzz has a lot of outdoor space photos that you can use for inspo!

I'd agree.....rethink the gravel surface. Pavers or flagstone would be a better and more durable choice of materials. Also, 55'x30' is a massive size for a patio or outdoor entertaining area. That's bigger than many folks' entire backyard! You could reduce that by almost 50% and still have plenty of room for the elements you wish. Having the patio usable 3 out of 4 seasons may depend heavily on where you are located, perhaps requiring some sort of overhead cover to protect against excess sun or inclement weather. Hiring a designer to assist you with this project is probably a smart thing to do.