As a FOSS enthusiast and working in architecture myself, I agree that Blender is “not there yet”. For sun studies, it was there with the ODS plugin. For dimensioning and documentation it is far away, and for BIM it used to be far away, but now it is functionally more powerful than most BIM software (Revit / ArchiCAD / Tekla / etc), but just needs more streamlined workflows.
However, to a newcomer, it can be an extremely confusing and unstable environment. To help, I will write a tutorial on using the sun position addon for sun studies. I will let you know when it is done.
In the meantime, you can check out BlenderBIM - OpenBIM authoring with Blender and https://blenderbim.org/ - which I am developing and using actively in large scale commercial projects. I hope this answers your questions for the BIM side of things.
For construction documentation, there are a few of us trying to solve this, and it is a work in progress. I estimate a working prototype within a month.