Let us choose between LS/DS at least for Geo TB because they are different difficulty

You're just speculating. We haven't done the LS Geo TB yet so you can't really say it's more difficult. The opposition in the DS Geo TB are G13s. Just because the minimum GP entrance requirements are higher, that doesn't mean the opposition is any more or less difficult. The minimum GPs are there to force us to gear up more characters and prevent people from being able to get through battles with only 3 or 4 strong characters.