Herc Driver,
What you describe is an utopian way of designing a small N scale layout
In real life things goes like that:
The spouse says: " You are allowed a ... inches by ...inches for your layout ". ( please fill the dots with whatever your spouse, girlfriend or whoever you live with allows you to have )
Please note, I said inches, not feet

That automatically dictate the scale you'll be modeling in ( unless you like a HO switching layout ).
Knowing the layout size, you can now determine what industry
won't fit on your layout : forget about the big paper mill , the tall blast furnace or the grain elevator . You'll realise that the single spur on your layout is just long enough to serve 1 newsstand and 1 petshop
select what type of rolling stock. Are you kidding ! the maximum 9 3/4 " radius curves of your layout only allow you to run an Alco S-1 switcher and a few 40' boxcars ( without caboose of course ) . Your vision of an UP class 4000 big boy just ended right here.
Ok, OK I'm kidding here .
Herc Driver procedure for designing a layout makes a lot of sense, but imho the layout must fit the available space then the rest follows.
And I have to give credit to my spouse, she limited my railroad modeling space ( ambitions ) because I already took some space in our living room with my footie ( soccer in North America ) Subbuteo game. She is Brasilian , so she doesn't object at all with my soccer field.