Because the title question leads with "How is our tagging situation?" I'm going to start by addressing number 3. If you'd like to skip the detailed breakdown, feel free to jump down to the Comparison Summary heading.
How does our current tagging situation compare to similar sites like Astronomy SE?
To analyze the tagging behavior of Space ...
I am in favor of a firsts tag. The records tag should mean the greatest or smallest of a number (i.e. different from firsts). However, it would involve a considerable amount of re-tagging.
There seem to be too many questions for a how-many tag, so I am opposed to that.