Suggestion: for multiple selections, show missing tags in Extended Tags Panel

It could be very useful for many files. Extended Tags Panel, in addition to showing different tags, would also show, if any existing tag is missing somewhere. This way you could check at once, if you have all the necessary tags everywhere. You can then filter (or sort in order) to find, where the needed tags are missing.

You are right, this would be useful.

I check this situation currently with columns for every needed tag.
In your example, you sort the columns "ALBUMDATE" or "DYNAMIC RANGE" ascending or descending. This way you can immediately see which tracks does not yet have the needed information. This works very fast even with several thousand tracks.

Or you could filter for all the needed tags and show only those that are empty with something like

For now, i use another solution, a grouped filter: %albumdate% MISSING OR %dynamic range% MISSING OR %third% MISSING OR %fourth% MISSING... Then i check the results.