I really liked the first volumes of Hellsing, where the gore and the dark atmosphere were mitigated by all those jokes, and I am wondering if there is something similar (I also love the art style in the beginning, so if you know manga with similar drawings please recommend those too)
There was a dark fantasy manga called Bastard it had it comic moment but don't know if it ever was translated completely. All Rounder Meguru is a Martial arts manga lacks darkness but is Seinen and really funny (mostly when trolling the MC) and Eden - It's an Endless World! is form the same author of Meguru. Now for badass military Seinin action I highly recommend Red eye (the MC nickname is genocide and a well deserved nickname).