@unknownmat I share your same enjoyment of these types of novels. Death March was my first proper web novel and still one of my ongoing favorites.
I think there's a few factors involved that elevate these fics above other isekai novels. For one, the protagonist is mature and actually behaves like a responsible adult. Another aspect is that you can tell the protagonist actually enjoys helping others and takes pride in the development and growth of the students/children/teammates around them. The author also gives a sense of progress and accomplishment to the side characters.
Another minor aspect is there's often a focus of progress in not only skill/ability development, but also the equipment and territory. It makes sense why these fics focus at least a little on introducing modern knowledge and/or traveling to various places. Still, there's a few other unexplainable aspects that make these fics like capturing lightning in a bottle.
While not the same and not always meeting the criteria, I felt these fics to somehow satisfy a similar niche emotionally:
Dagashi-ya Yahagi: Setting Up a Sweets Shop in Another World
Making Magic: The Sweet Life of a Witch Who Knows an Infinite MP Loophole
Kuma Kuma Kuma Bear
By the Grace of the Gods