We're on the same page! My plan is to add the ability to load saved configs in the gui as well. If only because saved configs need a way to be edited (rather than needing to delete them and recreate new ones).
I haven't decided on the best way yet, but your combobox idea definitely warrants a look. I don't know how simple any of it will turn out to be either, but I'm up for the challenge regardless.
One problem I may run into, is that as people change the the available tags in TagMechanic's config dialog, that will change the combobox indices that are saved in the new config jsons. There will need to be some robust error checking for sure.