I've hidden most of the columns shown in the Tag Browser. It looks and feels strange but I'll see whether that helps (and then try re-adding some of them gradually). I'm waiting for the new Calibre release where columns with no items will be hidden automatically in the Tag Browser (as many of the columns are relevant only for one of my virtual libraries).
As far as the column icons are concerned: I have 6 simple rules (mostly with one condition only) for the Title column.
The things get more problematic as far as the icons in the Languages column are concerned. For each book, a flag is displayed, depending on the language of the book. If the book is a language learning material, the flag depends on the content of the custom #target column. For some languages not included in the rules, a generic flag is displayed.
I'm really fond of this solution
but the truth is it does require lots of rules (17 in total at the moment). Do you think this might be the biggest problem? (Before I introduced the #target column, I'd only had 7 or 8 rules for the flag.) I know, I know, it's not about finding one culprit - but do you think this still qualifies as "in moderation"?
(I also have 14 rules for column coloring, but here I could pretty easily get rid off some of them.)