Just a few thoughts that drifted across my mind this morning. I was thinking back about 10+ years ago (didn't realize it had been that long!) when I first 're-entered' the hobby not knowing a great deal about it.
I first purchased a used layout that a lady needed to get out of her basement. It was a rather invovled Atlas plan that her late husband had built the benchwork for and just started to lay track. I experimented with it and learned a lot.
Then it came time to sell that off and move to FL and built a new one. I had learned that I wanted to build it in its own shed rather than in any home space, and ultimately my shed size was going to be limited to 12' x 16',...a little larger than what you are thinking. Because of my advancing age, and experiences with that previous layout I knew I did NOT want to crawl under the decks,....so I chose what is termed 'around the wall' . That leaves the decks cantilevered out from the walls for easy access to wiring, and a multitude of other sins. I would HIGHLY recommend this approach.
Givens & Druthers: You might google this term and see the big number of suggestions as a way to go about determining what you MIGHT want to do with a new design. This list can always be modified as your planning goes along.
So here you can see some of my initial request for help with a new layout plan....on this forum
https://modelrailroadforums.com/forum/index.php?threads/help-please-design-a-dbl-deck-layout-in-its-own-hand-house-shed.29493/
You can ignore my desire for a double-deck arrangement and just look at the main deck inspiration, the Anon& Muss
https://modelrailroadforums.com/for...in-its-own-hand-house-shed.29493/#post-414043
What I thought you might find interesting is that it has 3 blobs,...perhaps one for each of your 3 towns?
Those blobs need to be able to contain 24" radius tracks which would be enough for your modern diesel locos. They were not enough for my longer steam engines.
City landscapes can take up a lot of real estate, so perhaps to need to consider making a lot of the buildings as 'flats and semi-flats against the corner walls. That is what I am going to do with my city of Balt in the one lower corner of my room. I'm even going to use some paper textured buildings on foamboard to preserve real estate
This was another layout design I looked at for inspiration when first looking,..
https://modelrailroadforums.com/for...in-its-own-hand-house-shed.29493/#post-414056