I think you touched on some important things here!
we as modelers often suffer from this track overload syndrome. yes, sometimes it works out ok, but, most of the time we tend to have WAY too much track, back to back yards and stations, etc. and too little scenery to create the impression of distance. but there ARE some scenarios where that is ok, so each to his own.
your suggestions of decide what MUST be there, what SHOULD be there and what WOULD BE NICE TO HAVE is absolutely critical. it connects with the TOO MUCH TRACK problem. we add additional track to add more industries etc, and eventually we have a crows nest of track.
Decide what is more important, RUNNING trains, or OPERATING trains. Although the 2 are not mutually exclusive, it will put some specific requirements on, for example, yard design if you want to operate.
We also need to decide WHAT IS GOOD ENOUGH. and do everything to that level, and no more. we often look at a scene, and decide we need to add stuff, that is fine, but keep in mind, the rest of the layout also needs our attention!
Building any layout is a learning experience, first layouts even more so. you tend to have to do things, and then, later go back and correct/improve/fix things. this is unavoidable. but you CAN limit this by talking to other knowledgeable people BEFORE tackling a specific task. for example, before making your rivier, talk to people that know, and get good advice, and learn from their bad experiences. this not only saves you time, but often LOTS of money!
And then, the last 2 comments I want to make...
1) this is not a race, DO NOT RUSH things. do it right first time. spend the time to get it perfect. it is cheaper and likely going to save lots of frustration later, if you get it right now. ESPECIALLY
TRACK AND ELECTRONICS!!!
2) ENJOY WHAT YOU DO!!! if you are not having fun, put down your tools, and come back later. the quality of your work will be better and as such, the long term enjoyment of your layout will EXPONENTIALLY INCREASE! yes, there are some crappy jobs, and for each this is different... I hate ballasting with a passion, but it has to be done. as a result, my ballasting work is not up to standard. but once done, I at least get it as good as I can, and that is good enough for me...
regards and enjoy