I am still pretty shocked that there’s still no all-in-one Apple-provided doodad to do this. And there are some articles but they don’t seem quite complete.
So, here’s how I did it for around $26, using two devices on Amazon:
This gets you from Thunderbolt 3 (via USB-C) to full-size Displayport.
This gets you from full-size Displayport to Mini Displayport.
I could’ve sworn that the USB-C-to-Displayport adapter I picked up listed Displayport Alternate mode and all that jazz, but apparently not. Either way, it seems to work for me (?).
Also wish it supplied power on its own – it doesn’t – but I guess I can live without that. Sucks though.