Hebi no Seikatsu
Not only bittersweet but TRAGIC. Chapters 1-3: Twins of Alpha and Omega grew up with their Alpha childhood friend but the alpha sibling had a crush on their childhood friend, unfortunately the heat took place when the friend was getting something forgotten from the classroom and so the omega was marked and probably pregnant so the alpha sibling was hurt and disappeared from their life until the funeral of the omega sibling where the alpha sibling returned and saw the childhood friend together with their child but runway, after few weeks he returned and accidentally saw the childhood friend at the tomb asked him to check out the deceased things, which among was unsent letters by the omega sibling. (T_T) Chapters 4-6:
Kodomo x Otona no Houteishiki
Chapters 1-2: teacher x single parent raising child. Chapters 3-5: child grew with his former bully turned best bud. Chapter 6: a guy who wants to be popular and handicrafts loving person.
Bokura no Shokutaku
I thought the child & the glasses guy will end up together
Yozora no Sumikko de
Sequel: Yoake Ni Furu
Oyasuminasai No Ato Wa