Only way R-Rated Superman works is if they say fukk it and do Darkseid.
Easy way to make the stakes obvious at that point is keep all the language/excessive violence to the villains, and literally run that bytch like a pared-down (cast-wise), amped-up(violence-wise) version of Infinity War with it really being Darkseid's movie and more than half of it is just him wrecking shyt, painting walls with women and children and Superman is out there trying to stop him.
It'd kinda be the only way to make MBJ work there, too. His range is limited, especially on the bluer side of the emotional scale, so he's not gonna be able to sell trauma and sorrow as well.
fukk it, cast Halle thick-ass sister Chloe in that bytch as Kara(Super or Power), let her get beat down, and then send in MBJ for that third-act heavyweight clash. Otherwise... this sounds like trash.
Woulda been all-fukking-in on him playing Duke Johnson for Batfleck though. That shyt woulda fukking worked for me.