I went through a planning stage similar to yours. This is my first permanent layout. I had a pretty good plan to fit the space I had available. Then we moved a year and a half ago right as I was about to start building it. Now the space is different and also much larger. I didn't have the budget to fill the whole space at once so I ended up just getting a 6'x16' Mianne kit and laying some track on it. It has given me a good sized layout to experiment with and I have learned some things about layouts by having it.
The next phase is to expand to a more final size of about 16' x 22' in an L with each part of the L being about 6' wide. Then there will be a yard about 2'-3' wide and 10'-12' long about 2' out from one of the L's. Also a similar size to yours. One of the reasons I chose the Mianne bench work was the ease of re-configuring it, which was part of the plan. I think I have a pretty good plan now and some re-configuring will be needed which should be very easy to do with the Mianne.
Maybe you could do something similar to what I did? With your space size already defined, and you probably have some idea of what you what to end up with, just get a table going and start laying some track. That might help some with the getting stuck on the planning part and get things going? I didn't know enough about permanent layouts to know if I had a good plan or not when I started, but I had an idea about the space I had to work with. That prompted me to just get a table up and start experimenting. It would at least give you the opportunity to run some trains while you work out the final details. Unless you are an experienced layout builder and have a few under your belt already, I think probably most of us go through some changes during the building phase.
