Is there any BLs where the main couple takes a really long time to get together, or like m...

Starting with a lie (slowburn dont get together until the end)

try reading shounen ai's

Heesu in Class 2

http://www.mangago.zone/read-manga/roommates_young_seok/?Xajs

Here U Are
http://www.mangago.zone/read-manga/here_u_are/?4wvG

also, there are other relationships that form with other characters in the manga. i'm pretty sure there are 3 other couples.

http://www.mangago.zone/read-manga/sekaiichi_hatsukoi/?Mpi8

Haha

Unintentional Love Story