Hi this is an office romance manga (i don't remember if it was rivals to lovers or not) but after they get together, one of the main lead has a short trainee under him who falls for the main lead but he sees him kissing the other main lead in a printer room and does some plotting against the latter.
This is a ship trope I found of which I'd like to read some mangas
https://twitter.com/greekfiires/status/1352407201328062466?s=19
In this manga (I think the last chapter) the couple wore yukata and went to a festival. They go to this forest and fuck but the uke had a candied apple with him and the seme asks him not to drop it. (That's all I can remember, some details might be wrong)
http://www.mangago.me/read-manga/boku_no_omawarisan/
I think it’s somewhere in here
Another manga with a similar scene is Blue Sky Complex. This chapter: http://www.mangago.me/read-manga/blue_sky_complex/mh/608dd5e2d7bed9c6/Vol3_Ch166_Special_Booklet/pg-1/
Here's the link : https://www.mangago.me/read-manga/on_doorstep/mf/c002/1/
Credit page at the top
He should be the beta from the second couple if I remember correctly.
https://www.mangago.me/read-manga/kashikomarimashita_destiny/