I don’t think I’ve ever seen brackets or parentheses used in the nomenclature for species complexes themselves (in the strict sense) and I think it would make things more confusing to implement them such that it’s an iNat-exclusive system. The correct notation I believe is “Type species-complex” which I think would aid in mitigating confusion if iNat adopted it (not “Complex Type species”). However, staff have said before that changing the naming system for each taxon rank would unfortunately be quite a big undertaking given the current system being used.
This is a point of discussion with the eventual introduction of genus- and species-groups as well as you mentioned. Instead of “Taxon-group” it will be “Species/genus Group Taxon” based on the current system. This is an issue that will definitely need to be addressed once things get moving, I think.
The issue with subgenera really irks me hahah—calling subgenera “subgenus _____” without mentioning the parent genus in the taxon name makes things confusing when there are multiple subgenera with the same name in different groups (same will apply to species groups). Staff have said that implanting the system of “Genus (Subgenus)” would be a pretty major change to the current way things are run and isn’t a priority at the moment, so we’ll have to see what comes of it.
edit: here’s the species group feature request, by the way
edit 2: and a feature request for fixing the notation for subgenera