So I was struggling with the same issue. Tag list showing 9k tags. Difficult to browse for some purposes, lots of scrolling
Now, the solution that works for me is to create a new column 'Genre'. Then, scroll the tag list for tags you want to be able to browse in the new column list. I simply looked at any tag that had lots of books in it. Then select all books from that tag, bulk edit and add that tag to the custom 'genre' column.
That way, I get to keep all the tags for searching purposes and simplifies my overview in a new hierarchical column 'genre'.
To create the new 'Genre' column you can easily find that on Google. Browsing through all the tags is actually quite interesting and for example, I came across the wonderful tag 'Space Opera' that I would never have thought of myself
Just posting this reply on an old thread as I have been searching a lot for solutions and it wasn't that easy to find exactly what I wanted to be done!
The last tip, if you start browsing your tags starting with 'Z', your new genres will always be added to the start of the column, which makes it easier to see what has been added already.
As one user "jacr" once said: "I weep for the poor unwanted data consigned to the big black hole of unrequited binary love"