The fact that people seem to be incapable of looking at this chapter in anything but a sexual light is a bit... you know. Like, for most of this chapter, they aren't acting like his girlfriends at all— They're going all in on the elementary school roleplay. + They specify at the end "We're not really kids." And we also literally have chapters where they become actual babies and no seems to complain about that. + Hahari all the time.
But, anyways, I do think we should get more chances to see adult Kusuri. Treating her as sort of separate from kid Kusuri seems like a good way to do that. You've gotta give all the members of Rentarou's Family equal love, after all!