Responses
Maybe https://www.mangago.me/read-manga/kisses_x_kiss_x_kisses/?
I might be subjective (if you go to Photos about this manga, the first 3 pages are all photos saved by me), but I think it fits the description, though it's more on the innocent side since it's shounen ai
It's more than flirting but... relationship guidelines IS the tension
in big need of coloured bestfriend to lovers manhwas with leads being in denial of having feelings w one another but the tension, the casual flirting & eye contacts are saying otherwise. wouldn't mind if it's a slow burn, just not too frustrating