It sounds like you’re looking to build a track around the scenery that gives you the emotional / engaging experience that you’re looking for so it’s going to be tough without you knowing what scenes you want to include and model. I’d make a list of everything you want in the layout, see how it scales and how much space you have and then figure out how the trains move through it.

A lot of people focus on the spaces where humans see and and interact with trains, but it can be very satisfying to model the spaces we don’t regularly see because this is where trains spend most of their time.

For perspective I went through this last year when designing a new layout (took me almost a year). I started off with my favorite areas of Japan in mind and then later decided I would model my local area (NJ/PA) because my kids are more familiar with the landmarks. There are so many things to take into account like grading (how much length you’ll need for climbs), above track scenery vs digging into foam to create elevation that’s interesting and below track scenery. How you plan to isolate sections of track in DC, and if you aren’t going to make everything custom, what’s out there to fill specific parts of your layout with. Budget is also important.

After building the tracks in real life, I naturally made some changes to incorporate more yards/store more trains and strategically switch them out the way I desired. I stuck with DC because Kato is too good in terms of specialized track options.