I have about 4,000 personal digicam pics that are only sorted by date in several subfolders so far. I want to use the category feature of XnView to finally have comfortable and quick access to any pics that I usually never found...

I found zero information about this feature in the online help or the forum FAQ. I worked with this feature and think that I understand how to use it - but it is so cumbersome that I can't believe I am doing it right. Ok, here is what I do:
Enter a dir with the browser, select first picture, then click in the category area at the bottom at the right category so it gets checked.
Problem one: how to change to the next pic? Too much mouse movement (too far away), and the keyboard (pic up down or similar) only navigate in the category area, but don't change to the next mini-pic for selection.
Problem two: when I change to another pic a confirmation dialog asks me if I want to save the changes (the category definition). Doing that after every single picture is

Problem three: I want to speed up my work by multiple selection of mini-pics in the browser that will get the same category. But as soon as I select more than one the category function vanishes.
Hey, I have 4,000 pics to work through! This way I will not finish before 2025

So what am I doing wrong?