Check out this post:
Alongside what it lists, I can personally vouch for this HDMI female to VGA male adapter: https://www.amazon.com/dp/B01B7CEOVK/?ref=exp_retrorgb_dp_vv_d
It's good to at least a 235Mhz pixel clock- I might test it for higher clocks at some point, despite my CRT not supporting anything higher, just to help contribute to a list of valid adapter choices. Unfortunately, most adapters seldom have DAC pixel clock listed in the specs due to how they're geared more towards crappy old LCD monitors and projectors more than anything. 1920x1080@60p, with its puny pclk of 173, is pretty much always the max for those things.
I thought the problem with the 10 series is that it doesn't output an analogue signal, which is why there is no DVI-I or why the VGA to HDMI cable I tried didn't work. I'm presuming then it is a converter rather than just an adaptor.
The USB-C output of motherboards is not good for video adapters because they usually don't support Displayport alternate mode and when is supported it works only with the integrated graphic of the CPU, so no way to connect it to your GTX 1060.
About the adapters try with the StarTech DP2VGAHD20, it has been tested by Flybye and seems to be a very good DP adapter with a pixel clock of 375 MHz, if you want more try with one of the Synaptics based.
Why are USB solutions suggested then?
Forgive me for my lack of knowledge (my background isn't electrical engineering), but 2304 x 1440 at 100hz or anything just over spec (as I don't really want to stain my rare and valuable piece of equipment) will be no more than a pixel clock of 350 MHz so as long as the converter can do that pixel clock it should be fine.
Also I'm in the UK and that DP one you suggested is quite expensive compared to some others I have seen.