1) Why does it NEED to be over? What about all the problems that pop after they are a official couple? There are plenty of series, there is seinen stuff where you just see what couples do, again the author has the experience to know what happens after that, she could write it.
You know what is a chapter I really liked in Sweat and Soap? (Seinen I know) where they are looking around for a house to find, it felt grounded, something they were doing together and perhaps with some interesting points to think about when shopping for a house.
2) Oh really? Well, let's see, Exhibit a) all those scenes where she imagined doing the deed were quite graphic.
exhibit b) You might be right so I looked up where this is published "Young Magazine the 3rd" then I looked what else they published, I picked the flashiest one "Isekai Hitokko Dōbutsuen" finished in 2022 after 5 volumes. Not exactly kid material, check the tags
https://www.mangaupdates.com/series/urckrq1/isekai-hitokko-doubutsuen
I also checked the Raws, it can get dark,
sexually.
Though, doesn't look worth reading.
3) Mangadex says the demographic of this is Seinen too.
So I don't think those obstacles exist.