Mmmh, so him saving her life giving her a Coke mouth-to-mouth increased her level cap... but that doesn't really mean that having sex would do the same.
I kind of don't understand where the author wants to go with the story... In most Isekai, the problem is that they don't really have a well defined objective.
In this one, he has too many. Visiting the other world, getting it on with two beautiful girls, getting lots of money selling gold coins, increasing the level cap of his harem, becoming the dungeon master to get eternal life, exploring the effect of stuff from Japan.
None of those goals are really mutually exclusive, but spreading the story too wide might be bad overall.
In stories like Re:Monster, Kekkon Yubiwa Monogatari, Overlord, Re:ZERO, Tate no Yuusha, ... the MC is usually going after one thing in particular, then other things are introduced little by little, but we can see the MC completing his goals little by little.
In this one, he has no goals for 5 chapters, and then all the goals at once.