The only one I can think of is not a manga but a novel you can read for free on kindle unlimited. I'm not sure if you can read it else where but its called Bro and the beast by L.C Davis LMAO. I'm not sure if this is what your after but what you described is exactly what happens in this story. I hope this helps and if not I hope you find what your looking for!!
Yasss I think that's the one
BTW is it smut?
Okay great!! and I think theres a little bit, like one or two scenes each book.
anyone knows where i can read the novel about a "alpha" dude gym bro who got transmigrated into another world as an omega (lol)