Basically, I'm looking for Mangas where humanity as a whole absolutely dominates and is potrayed as awesome while doing so.

There's no Warhammer 40k mangas to my knowledge, no, but there are a few Halo books written by Japanese Authors...

Some of the folks on Reddit's /r/HFY might have a few suggestions. Off the top of my head I'd say that GATE: Thus the JSDF Fought There would fit. I know there's another one in my follows list (that hasn't been updated in a while) with the premise of the entirety of Japan (islands and all) being transported into a classic high-fantasy world and vastly outmatching the locals, but I don't recall the title off the top of my head (may come back with the title later if I can remember/find it).

does this fit what your asking for, https://mangadex.threeproxy.net/title/29837/nihonkoku-shoukan

only 10 chapters but I'm enjoying it so far.

Gantz