I searched the forum but I was not able to find a simple solution to the problem of tagging multiple files so I make my proposal:
when you select multiple documents, the Tag bar, instead of reading “Multiple selection”, could show the TAGS common to all selected documents (if any).
You can then delete one of the common tags or add a new common tag for all the docs simply writing in the bar.
I hope to see that (or an equivalent simple alternative) in the next release. I could then stop using other apps to manage my files tags.