Some update with pictures:
Implementing F51 - concentric ellipses in Perspective Ellipse Assistant:
Except that it seems like at some point the ellipse stops being an ellipse and becomes a hyperbole. In my current code, it just stops growing, but it’s very jarring for users, so I’ll have to implement the hyperbola part too 
And possibly even mirrored image on the other side of the horizon, too… so, I guess 3x more work than I expected 
(Biggest ellipse below:)
Will post MR link when I create it.