Just as a reminder, knowledge transfer, because this is easily overlooked in testing but at least one customer would have gotten very annoyed if we had ever deployed an "Update all certificates" function which "added" new certificates. Even with the update of a single cert, we had a "funny" issue, like if you had expired certificates from anywhere and not from WKD (which old keyrings have a lot, maybe with many uids). Suddenly an update would pull in new keys which come from WKD but maybe there they all only have one UID. Because for keyservers the identifier was the fingerprint and for WKD the identifier was the userid.
Or even worse, you explicitly threw out the OpenPGP keys from WKD because you wanted to use only S/MIME, then such a function may not search on any OpenPGP Sources.
When I worked at Kleopatra we didn't want such a feature in GnuPG. Our strategy was to update keys when they are used, about to be used or close to expiry. The whole locate-external-key thing.
I think the feature we had to update in the certificate details is good. But i recommend especially keeping the S/MIME / OpenPGP difference in mind. I would also call it "Search updated certificates" with a tooltip that it might also find "new" certificates for the user. And then an option to disable this either for S/MIME or for OpenPGP.