Before trying this: give it a thorough testing on a variety of copies of your music!
You could use a script like this, that will prevent Picard writing these specific tags.
I believe the list is complete and correct, and contains all tags that Picard could write, but I am not 100% sure.
(maybe somebody could confirm)
Make sure to remove the entries from this list that you do want updated/written.
(and obviously: make sure that under Options > Tags you do not have ‘clear existing tags’ checked)
$unset(album)
$unset(albumartist)
$unset(albumartistsort)
$unset(albumsort)
$unset(arranger)
$unset(artist)
$unset(artistsort)
$unset(artists)
$unset(bpm)
$unset(comment)
$unset(compilation)
$unset(composer)
$unset(composersort)
$unset(conductor)
$unset(copyright)
$unset(originaldate)
$unset(originalyear)
$unset(date)
$unset(discnumber)
$unset(totaldiscs)
$unset(encodedby)
$unset(encodersettings)
$unset(engineer)
$unset(genre)
$unset(grouping)
$unset(key)
$unset(isrc)
$unset(label)
$unset(language)
$unset(lyricist)
$unset(lyrics)
$unset(media)
$unset(mixer)
$unset(djmixer)
$unset(remixer)
$unset(mood)
$unset(originalalbum)
$unset(originalartist)
$unset(performer:*)
$unset(podcast)
$unset(podcasturl)
$unset(producer)
$unset(discsubtitle)
$unset(subtitle)
$unset(title)
$unset(titlesort)
$unset(tracknumber)
$unset(totaltracks)
$unset(work)
$unset(website)
$unset(license)
$unset(acoustid_id)
$unset(acoustid_fingerprint)
$unset(asin)
$unset(barcode)
$unset(catalognumber)
$unset(discid)
$unset(gapless)
$unset(musicbrainz_artistid)
$unset(musicbrainz_discid)
$unset(musicbrainz_originalalbumid)
$unset(musicbrainz_originalartistid)
$unset(musicbrainz_recordingid)
$unset(musicbrainz_albumid)
$unset(musicbrainz_albumartistid)
$unset(musicbrainz_releasegroupid)
$unset(musicbrainz_releasetrackid)
$unset(musicbrainz_trackid)
$unset(musicbrainz_trmid)
$unset(musicbrainz_workid)
$unset(musicip_puid)
$unset(musicip_fingerprint)
$unset(releasecountry)
$unset(releasestatus)
$unset(releasetype)
$unset(script)
$unset(show)
$unset(showsort)
$unset(writer)
And did I mention that before trying this: give it a thorough testing on a variety of copies of your music?