To acheive the right amount of donk in the badonk, you want to be able to control the first two, whilst looking at dat rump in the latter two... so through the game camera in ge lighting conditions.

So... how?

Okay so first, lets talk volume- the low mesh.

Create the plumpest personal pillows you can, and really push it.

Now copy the model and make the sort of flat ass homophobic straight men would sculpt / tell artists to change.

Making models a blend shape and import it into the engine. You now can tweak the value in real time and check out the back end development.

Once you are done, bake in the value and reimport to optimise, or leave it and add it in as a fun easter egg or unlockable.

Now the normal map needs the same treatment... normals can be scaled up and down, using nodes in the shader.
To get the ideal values for a local area, (like a man's glute-a-toots) mask the bum using vertex paint (say, red for that spanked look) and in the shader...

...use the red channel to LERP between two versions of the normal map- one of which you run through a flatten normals node (unreal).

Now you can ramp that up and down and you have a plumpener value. Again, you can bake this.

Ass perfection in real time? What is this sorcery.

Does your anaconda want some now hun? OMG Becky, buy me buns*

ko-fi.com/dellak

*bread.

I suggest that you apply the same jiggle physics straight men put on boobs to video game mens butts.

But seriously, the next thing you want to do is get the volume on your posterior as the character is moving.

To scrunch that man muscle right, you should think about how the underlying muscles work.

Note there are upper and lower glutes. Upright, the lower glutes are plumped, forming two subtle masses.

As your artificial man moves his legs, the glutes activate and spread under. This gives different shapes than if you weight all your verts to just the hips and legs.

See how the maximus flattens as it goes slack, as the leg is raised. It pumps and expands as it contracts, as the leg straightens.

You can see this in action using ballet dancers, as they have tights. Strangely, all men mags butt workouts have the men wesring very large, loose shorts for the same reason the publishers will push back on your nice ass modelling

To acheive this, you have to think of each cheek as its own entity, and give it a bone that can be adjusted to correct the butt. For the most part it can be driven by the leg, but you should be able to key it so you can custom tweak it. Place it so scaling works

Ultimately you cannot experience the true power of your model's ass unless it is in engine, and in motion.
That requires your pipeline to let you tweak in engine and have a fast turn a round.

The end

One more thing... I am a gay woman. I have sculpted kr hand painted and modelled a lot of male butts over my last 2 decades in games. Look at it this way, if a gay woman can do it and remain gay, a straight man can do it without fear of losing their precious het masc cred.