I would replace the railroad tie retaining wall when you are making these changes. It does not look in great shape. If you want more useful feedback, it would be better to have a more complete view of the area with the blue tarp rather than just the edge of it. Additionally, how do you plan to use your outdoor space?

We redid large segments of our backyard in the last few years. Pavers, concrete, and decking are all pretty expensive, so you want to make sure that you will use what you are installing. One of the best pieces of advice we got from our contractor is to really think about how we will use the space rather than just putting in a patio that looks good that might not fit our needs. It took us a while to figure out what we really wanted, but we are glad that we took the time to really analyze how we entertain and how we would use the space. Last weekend, we hosted 40 people on our patio, and it worked out just like we anticipated.

More photos would help show slope of the yard and where the doors area. Is there any reason not to level the place where the trees were? Then you could remove the retaining wall.