...up that high the sharp edges should not be a safety problem.
In any case you always can make the edge blunt - Kamm design will work as long as the air gets a clean break from the surface - so the edge of the upper rear end cap has to be as sharp as you dare...
The basic idea is, that the air leaves the rear edge without a chance to "follow" the surface. Rounded edges ALWAYS drag some of the air around the edge curve (koanda effect) and finally create turbulence.
The soft downwards curve of cars at the rear is just a design compromise - Kamm didn't care if it goes straight back or slightly down (...it seems the max angle you can curve down is 11° - after that air would become turbulent (as opposed to laminar flow) and create more drag.
There is plenty of reading and looking and studying on the net, regarding vehicle aerodynamics - you wouldn't believe how much difference it makes to point the exhaust
correctly - from actually getting some thrust from it, ....or at least zero drag, to acting like a bloody parachute - straight up stacks are a big aerodynamic no-no!
Then - how often do you
go at full speed and for how long? Not likely that any of the buses on here do even 1% of the average Long-hauler distances.....
Also, IF you plan to put anything on the roof - solar panels, roof deck, water tank, roof cargo pod, etc, ...your airflow is turbulent LOOOOONG before it ever hits the rear cap!
However - looking that sharp, I'd stick a few vortex generators on it, just 'cause!!
[But don't use the huge ones that are used to streamline long-haulers, too big and ugly!!]