Ah, a correction, I'd also have to run
DELETE FROM wp_terms WHERE term_id IN (SELECT term_id FROM wp_term_taxonomy WHERE count = 1 )
Or else that would only take care of the tags present in zero posts.
And I just discovered the database was so huge it couldn't be properly backed up anyway -_-
Would someone know, please, if that's the proper syntax ?
Thank you very much :)