I’m still not clear why this is “clearly inferior”. I presume you are happy with “Sonata”, so why not “Cantata” (or German equivalents)? Even Bach Digital does this. The distinction seems arbitrary to me. (and, in fact, both Sonata and Cantata are specified work types in MB).
An improvement in the “work type” field would be welcome, but I suspect that is for a different topic.
This has some attractions, although I’m not sure how searchable it is. In fact, I was thinking about adding a disambiguation tag to Classical Extras, which could either be separate or combined with a work tag (again, a separate discussion, but I would welcome views on this - at least it would allow a Picard user to show the information how they wish).
I have a bit of an aversion to introducing more conventions that require careful adherence to a specific format. The example you give actually has [Choralbearbeitung] in the disambiguation field, not in the main name, which results in it showing as ([Choralbearbeitung]) - not what you intended, I’m sure.