The width in your legend columns do seem quite exaggerated for a 0 pt setting. This is not something I can replicate, although I may be using an older version of Pro than you. I also know the latest versions of Pro was experiencing some buggy behavior with legends and perhaps this may be a part of the issue?
Here's a screen shot of a similarly setup legend to yours, with one main exception seeming to be that I do not include layer names:

You may notice somewhat of a gap between the first and second column. I believe this is due to the length of the longest label in other columns. Although there are different columns, the labels are being treated as if there is one column. If I turn off the "Equal column widths" option (which you have checked/turned on in the arrangement/fitting strategy column, this is the result I get:

You can see I get a greatly reduced space between the first and second column. Perhaps this may be the solution you're looking for?
*Edit: You may also want to try using "Adjust columns and font size" in the fitting strategy dropdown so the font size itself will get adjusted as you adjust the size of the legend.