Never trust Otto (anything, not just metadata downloads).
Doing that is pretty much resulting in GIGO.
Your end goals are similar to what I did.
I have a #usertags column where I log which source (store) and I have a #MstrFmt for the starting exact format (A store is not always a single format)
My original start was my 900 paperbacks (empty books. Metadata only).
I had a barcode scanner (only 60% had those) and I would do batches of 20 using
Add from ISBN (the scanner was Keyboard wedge, so those scans landed in the entry box as if I had typed them), I then resolved errors and not founds while the copies were on the table. If I did not like or get a cover, I flopped the book on my flatbed scanner).
Fix as you go, not let it pile up.
It took me almost a year to get that task done in my free time.
I fixed Author names (consistency, not always exact as on the cover. Not all publishers did it the same for a single author. Fn [M] Ln) I also used the Cover Author name (might be a pen name) and logged the real name in #psudo4
I also learned that Tags downloaded is a total mess, and did weeding.
Do we need 23 ways to say Science Fiction (or other genre)?
Be ready to Move from the {title} Series, other odd junk to where it fits in Calibre (or the bit bucket). Same goes for tags with a Series, Author... .
Sometimes Calibre just splits tags in the wrong place (Adams, Joey becomes 2 tags because they put a character in as Ln, Fn)
[Plug]
Quality check Plugin is your friend to help keep things under control.
Have a PLAN when you need to break an unrelated (Series) Tie (I chose series (Author initials) <
Calibre mostly ignores the (contents) when sorting , so I use this
feature other places as well.
Things to consider: Calibre only allows for 1 series and 1 series Index. No [5-9]. My choice is biased towards the Cover. I also use hierarchical series and Tags. eg Pern.Dragonriders That helps keep universes together Or tags Primary.refined (Fantasy.Urban)
Then there is what I call OBVIOUS. Do I need a Fiction Tag for Fantasy or Science FICTION
Rememer: Your books. Your Calibre... Your Way