tbh the "resolution drop" trick is solid but if u want to be super scientific about it, u should try checking the actual frame time graphs in a tool like Cap# or even just a basic overlay. sometimes average FPS lies to u cuz of micro-stutters.
if u see the frame times (ms) staying flat even when the GPU usage is low, that's your CPU screaming for help. but also, keep an eye on your RAM speed and latency!! ppl always forget that a slow memory sub can bottleneck a high-end cpu just as easily as a weak chip can a gpu. in a setup like mine, i noticed that jumping to a faster cpu didn't do squat until i actually addressed the memory timings.
also, for those strategy games u mentioned, check if they are thread-heavy. some older engines are basically just a single-core nightmare, so no amount of 16-core monsters will fix it lol. just a heads up before u drop a ton of cash on a new platform!!