Thanks for those answers.
I'm leaning towards the problem being single core cpu performance (I expect the logical chart shows one core at 100%), and not enough ram (presumably this game is a ram hog because usually 16gb is enough, or maybe there's other stuff hogging ram).
I didn't think it was the gpu because of low utilization, but have no problem recommending someone replace a GTX 1060.
So I guess combined we're saying replace everything... which blows the 200-300 budget.
Usually in this situation I recommend saving up for a complete pc instead of buying cheap upgrades for an older system.