Physics simulation is kind of unstable, maybe not even that, but its easy to do mistakes and get nice looking but not correct results from it.
My point is that while there are very accurate formulas for ballistics, physics in unreal is not always accurate (for eg. low/hig fps changes results).
So imo only one accurate solution is to trace this in real time.
Theoretically if you increase speed 10 times and increase force pushing down you should get quite accurate results 10 times faster.
And those results need to be updated only if player changes elevation so you could trace just 2 curves one below one above current elevation and store results in array.
There is even more optimizations you can do, you do not need to trace them every 1/10th of deg, you could do it like every 5 deg or so, this all depends on distance and accuracy you want get.
Or you could just trace such curve every 1deg and store all results in array
Then play those results for shooting instead of physics.
This way you always get accurate and consistent results.