Responses
Most of the following may not be very seinen or fit what you're looking for at all, but I'll put it here just in case..
I dunno if it was well handled but "bungou stray dogs"
oyasumi punpun (classic)
lookism
happy sugar life
Bokutachi ga Yarimashita
Jagaaaaaan
vinland saga (no but this is beautiful)
I wanna read something that has a lot of mental issues and real world struggles but also insanely seinen