My theory about the 3 year cycle still stands, and despite rumors turning out to not manifest, it's now been three years, I guess that means it won't be long now for the refresh.
At least your iMac still works, well enough to be online. It's a waiting game is all. By the way, Randy didn't want to wait, he bought it because his systems were falling apart. He's loving it. So even if you get the 2020 one, you will be happy. But yep.. i'd wait for the 2nd silicon - I can't imagine it will take much longer.