I’ve suggested a new image type to specifically identify tagging images (https://tickets.metabrainz.org/browse/STYLE-845), but I have no idea when it might get to the front of the queue. I suspect that the style change to create the new type is relatively straightforward, but I am almost certain that the database ramifications and code changes within Picard to make use of it are not trivial, so I understand and appreciate the caution being taken before making the decision to implement such a change.
In the meantime, I suggest that you might want to have a look at the artwork available at fanart.tv as an alternative. The artwork is quite high quality and the album covers come in a standard 1000 x 1000 pixel JPG file, with no file larger than 1 MB. There is also a fanart.tv plugin for Picard and you can set it up up to check for cover art from fanart.tv first, before checking other sources such as CAA and local files.
That’s what I’m using as I (slowly) go through organizing my music collection. On those occasions when there isn’t a cover already available from fanart, I try to prepare and upload one from a scan of my copy of the CD. The first ones took quite a few tries because the quality standards are pretty high and the moderators do a good job of upholding them. Having said that, the community and the moderators there are pretty helpful and I have learned a lot about image processing in a fairly short time. I now have almost 100 images that have been accepted (which is still next to nothing compared to most of the contributors)
I’m not sure how well fanart is stocked for classical music images because I haven’t started work on my classical collection yet. Because of that, they may not be much help in the specific case of your edit, but could be of value on other releases.