All “Spotify downloaders” you’ll find actually fetch from YouTube
That’s not quite true. There are tools that let you download directly from Spotify (e.g. Soggfy) and with a premium account you’ll get 320kb/s OGG files out of it, because that’s what Spotify uses internally. But since almost anything is also available on Deezer, downloading a lossless version from there is usually preferable.
You can of course remove the metadata, but you can’t really be sure you removed all watermarks hidden in the actual content, unless you can make two downloads from different sources have the same hash with whatever method you’re going to use. That way you’d know for certain that you caught whatever was inserted to identify you. Anything other than metadata will be very hard to find and remove in an automated way unless you already know exactly what you’re looking for though.
That said, this is how I’ve cleaned up metadata in batches of PDF files using qpdf and exiftool in the past:
for file in *.pdf; do
exiftool -all:all= -overwrite_original "$file"
qpdf --linearize --replace-input "$file"
done
Have you asked the operator of those sites if they are fine with this?
It would be a shame if they were to be taken down because of people scraping the site causing too much traffic costs or something.