Its less likely to be your hardware's fault but rather the game or maybe the drivers, I have an older system running a 4770k and a gtx980 and all is fine, but an older system means there is a bigger chance whatever bug or issues that might happen have already been fixed. For your case now, low CPU usage is normal, its quite rare that a game might make full use of the CPU especially when you have a beefy one and the 6700k is no joke, the 2070 super is quite powerful as well and it should be there no bottleneck at all that's for sure, you running the witcher 3 perfectly fine proves that point. Although, the 2070/super or the RTX lineup in general is still kinda new, and Nvidia is releasing new drivers and hot-fixes every so often, if its not a driver issue Id assume its some lack of optimization for the newer hardware meaning its game/engine fault, if you have a second screen you can live monitor your hardware while running the game, or through some charts or logs from afterburner and rtss, I so hate that bungie blocked the rtss overlay, it only makes troubleshooting harder. A good way to find the issue is to ask someone with similar hardware about their experience, if all is fine then the issue is your system, otherwise its a game issue or maybe some driver issue that Nvidia didn't fix yet, can be many things but since other games are running fine the hardware should be fine, software tho, you'll have to do some research.