Huh. This manga has ultimately gotten pretty scattered / lost focus. Hopefully, it finds it's direction again as the war prep comes into focus, but I don't have high hopes given it's slide over the last 20 chapters.
And I wish the author had just made the girls several years older rather than this 14-16 crap (after the time skip, they might be 15/16 now, with him 26). Or have made it so when he goes back, time progresses faster on this world than Earth, allowing them to 'catch up' to him some and make it less gross. Would also add some dynamic of risk to him going back and getting more stuff. Maybe it could've been a slight difference early, but the dilation grows stronger the more earth tech he brings over (as opposed to when he just teaches stuff). Well, whatever.