Since the graphic is isometric, it seems you don't need a too realistic drawing. In that case, I would use Inkscape. Inkscape it not a 3D tool, it's a 2D tool. But as you'll see you can get a 3D-like result.
Inkscape is gratis and creates vector graphics that have good quality and can be exported to pixel graphics.
Create a rectangle and single click it a ...
You can certainly solve a camera position of a single photo with known 3D data (anywhere from 4 to 7 known 3D points depending on what you know about the camera). PhotoModeler can do this. But to do this automatically with little to no user intervention on a scene of unknown content would be difficult.
You are describing Blender3D .
It has a top tier GUI for manipulating meshes, rendering, animation, texturing and so on, and all of the parameters for everything is Python scriptable - with the hints in the GUI itself spelling what calls are needed in script to manipulate the desired attributes and properties in the objects.
Once you get the grasp of it - ...
I was going to say Blender in a comment but you've mentioned it already.
Ease of use is most important. I tried blender and was blown away by the interface.
Blender can also be run almost completely with Python, if you are comfortable with Python then you almost never need to use the GUI or touch a mouse, which I also find intimidating.
update: I ...
Starts off right off the bat with a CUBE.
Tap TAB for Edit mode
Hit the number 3
Click on the TOP surface the one we will extrude now.
Hit e for Extrude
Press the number 2 key and ENTER which will indicate 2m in height since that is the default.
Move the UP and a new one with come forth.
After you have a few of them.
Now SHIFT click ALL the ...
I would suggest trying Houdini. You can create an inflatable balloon object out of a mesh with any properties you desire. There is actually a beginners level tutorial that shows how to do exactly what you are talking about. An apprentice edition with very few restraints is available for free.
Blender isn't a program well suited to engineering type creation. I'm unfamiliar with Unity, but I suspect the same applies.
For engineering applications, you'd want a CAD type of program. Fusion 360 is one such package, free for hobbyists. Another is OnShape, a web-based program. Both programs work well for highly technical model creation. As is common ...
I started off 3D modelling using this free online software called Tinkercad (https://www.tinkercad.com/). You're able to export to .obj .stl and .svg. It's really simplistic and great for beginners. It is also developed by Autodesk, one of the best-known companies for their CAD software.