I think the best/cheapest solution would be some rumble grooves 2-300 meters before the intersection to help alert you to it.

Traffic control devices that "alert" drivers and rely on their choice to slow down are pointless IMO. Better to make it almost impossible to drive at an unsafe speed without ruining your car, removing the choice.

Rounding off the corners of four productive farm fields is not going to be easy/cheap/popular, especially in areas where large farm equipment needs to use the highway too. And a 200m speed zone will not be respected by much of anyone.

> Replace the T with a roundabout

On the side of a hill? That's a ton of earthwork.

Country lanes don't get roundabouts. There's no money for that.