Spread the load MUD/road tyres

The grass down at our yard (horses etc) is on top of the most greasy clay soil imaginable. I have to tow a heavy roller to flatten the grass poached by horses and a pair of heavy chains on the rear 235-85-16 tyres are not a significant improvement. My 1994 Defender still digs in. My thinking now is to have as wide as possible tyre on low pressure to maintain traction by spreading the load. So can anyone recommend a wide mud tyre to get but still road legal? My road use is tiny so road noise is not an issue.

If you fit flotation tyres you will just sit on the top and go no where , if its that bad that 4wd with rear chains isnt getting thru then you need to stay off it , even steigers and challengers get stuck if conditions suitable .