So I just set my Vega 64 to Balanced, loaded the game (which I haven't played in a while), and set it to 1920x1080 with everything on ultra (even Hairworks). I saw the frame rate drop to the mid 80's while fighting a bunch of Renegades, but otherwise couldn't really get it below 90 while just running around. At my native resolution of 2560x1600, I never saw it drop below 60fps. In other words, I couldn't replicate your problem at all.
Low GPU usage is typically diagnostic of a CPU bottleneck. Simply taking note of overall CPU usage isn't useful.
Windows is a bit funny in how it reports CPU usage with multiple cores. In Linux, a process with 100% usage is running as fast as a single logical processor can. If it's using two fully loaded threads, it would show 200% usage. All the way up to the max, which would be 800% with your processor (8 logical processors). In Windows, however, the usage figure is always 100% or less. If a process is going as fast as it can on a single work thread, the percentage shown is 100 divided by the number of logical processors. In your case, that means a process showing 12.5% usage in Task Manager is actually maxed out on a single core. If it maxes out two threads, it will show 25% usage. And so on.
That's why I'd recommend downloading Process Explorer from www.sysinternals.com. It's technically third-party, but written by a Microsoft employee.
Star Process Explorer, then launch Witcher 3. Set it to Borderless Window mode in graphics. Switch back to Process Explorer, find the witcher3.exe process in the list, then right click on it and select Properties. In the Properties window, select the Threads tab, then click on the CPU column header until it's sorted in descending order.
The game should be running in the background while you're looking at the Process Explorer window. What you want to look for is the top thread in the list being reported as using around 11.5% or more of the CPU. If that's the case, then you're likely dealing with a CPU bottleneck.
If that's what you find, and you're not already doing so, you can overclock that CPU to get probably 10-20% more performance out of it, so long as you're using a decent CPU cooler.