The first task I would do is buy a new external drive and make an archive backup. Something to tuck in a drawer in case something goes wrong with the dedup.
I would suggest don’t rely on Recording MBID alone to do the work. Sometimes you get the “same” recording slightly different. Example would be a live gig reissued. Slight different timings of the end of a track may catch you out. It can be a guide, but I would not trust it fully unless Recording MBID and Release MBID are the same.
Not really relevant when you want to deduplicate. I’d trust the tool designed for the job. Picard does tagging, Dupguru looks for duplicates.
As with anything, grab a few folders of test data and try the tools out. Working with data where you know the expected outcome will mean you can see how good they are. Dupguru has a pretty geeky GUI, but it powerful once you get your head around it. Especially as you know you can mess around with it just looking at files without changing anything until you are sure.