I saw a recommendation in another Transmogs forum thread for this addon:
https://www.curseforge.com/wow/addons/better-wardrobe-and-transmog
The feature list of which includes a lot of improvements in sorting and displaying pieces and sets.
Looks like it’s being maintained (most recent update about a week ago).
I don’t use it so I can’t vouch for it, but the writeup sound promising.