the story about you x me
semantic error
truth or dare
spooky love tale of gongchan seol
park hanhoo's manager
https://www.mangago.zone/read-manga/spinach_bouquet/
https://www.mangago.zone/read-manga/19_days/
https://www.mangago.zone/read-manga/snail_pond_robber/
https://www.mangago.zone/read-manga/for_your_love/
https://www.mangago.zone/read-manga/the_sound_of_your_voice/
https://www.mangago.zone/read-manga/fools/
there's more but here are my top recommendations :) have fun reading <3
Monochrome Rumor is the slowest burn that i know
Shape of Sympathy and Form of Love are also very slow. They are technically yaoi but have r15 versions.
Slow burn shounen ai