That is completely up to you. But as you get the data from ALBUMARTIST - a field that most players use to keep an album together but hardly any player really reads and needs MIXARTIST and PUBLISHER I do not think that it is a good idea to delete ALBUMARTIST unless you have alternative data to set into ALBUMARTIST.
Or what do you intend to do?
For the discussion on easiness: I would use a filter to identify all the tracks with an opening square bracket and apply the action "Guess value".
And then I would invert that filter and treat the other files.
I assume that this will happen right now once in the lifetime of the existing files. So any kind of possible but rather elaborate and therefore difficult set of actions with conditions to match would be much less easy.
And all future files that you get won't need to split ALBUMARTIST into several fields as you would enter the data straightaway into the correct fields.
So, as you said that
where the 6th did not match the original data pattern, I would say that your problem is solved.