I don't think you are overthinking, I believe you are on your way figuring something out here. Developing your own system of methods.

I want to share some thoughts regarding your first question.

What is the shape of a project? It's "gestalt", it's structure?

It consists of three elements, and all those elements are repeated in the structure within. Thus, it is a fractal.

Let's look closer. The three elements are:
1) A purpose. A reason for movement and drive. In a way this points out to a location outside the scope of the project itself.

For example, the purpose of project "earn money, go to work" might be: sustain myself have money for fun. That purpose sits on a higher level than the work itself. You don't do work because of work but because of money. You do things in life to archive something, some state on a higher level then the level these activities reside.

2) Succesfull outcome: how will this look like when it's finished? Now, this is basically the same thing, the purpose of movement, but seen from inside the scope of the project. It's on the same level. You work to get the work done.

Similiar, on the GTD-higher-levels scale, the 50,000-feet purpose of your life looks from outside on your life, whereas the 40,000-feet vision asks "how does this play out in my life". You can scale up the model until your whole life becomes a project with purpose, outcome and action.

A successfull outcome is a goal. You can formulate each successfull outcome of your GTD-projects a a SMART-goal if you like. However it remains the same: each SMART-project will keep it's purpose and you will have to accomplish a set of actions to reach there.

3) Last we have Actions to accomplish the project. These actions kinda are the matter of the project-universe. Each project consists of actions.

Now, all this may sound a little crazy but this is due to my limited capabilities of expression in english language. I mean very concrete things here.

Let's look at an Action and it's gestalt. Each action looks like a project in itself! There you have all the three elements repeated, and that's why I want to refer to it as a fractal. Each Action has a purpose (which points to something outside of the Action itself, namely the project), a successfull outcome, and actions: the tiny steps you do to accomplish the action.

Now, if you want to, you can scale up and down as you like. If you are doing GTD you already did this. How did you define the scale of your projects and actions? Nearly arbitrary, out of your gut, right? You just cut it somewhere, where it makes sense to you. If you want to have several layers of projects and subprojects worked out, just go ahead. You don't have to call them goals, or objectives or projects for that matter. This is just methodology.

What would be the best number of such layers? From experience and what I read on the information superhighway I would say, have three. Open-ended longterm goals, the corresponding SMART-Yearly-Milestones and one GTD-Projects level beneath. But this is just my opinion.

If you've read until here, I hope I could kick your synapses into activity and your emotions into a state of joy. Happy projecting!