I suggest that you do some research into how Sepp Holzer does terraces.  He is kind of the terrace expert.  He talks about how he isn't able to create a road, without tons of legal mumbo jumbo.... but he can create a terrace that he can drive on to harvest with no problem.  This sounds similar to your situation.  Both terraces and keyline are designed to increase the length of time that the water stays on the land, and they do it differently.  Keyline takes a ton of investment, as the keyline plows are not cheap, and take a sizeable tractor to pull them.  One of the best things about keyline is it's planning.  It is brilliant the way that they plan where to put a road, and for what reason.  Also, keyline is about creating water reservoirs as high up in elevation as possible, and using that water throughout the property.  I have no doubt that this would work perfectly with terracing, but I would think of keyline more as a planning tool, and then terrace and do swales instead of actual keyline plowing (unless you have access to a subsoil plow)

Keyline has worked successfully on large scale properties in Australia. Generally in drier areas

You can find the full PDF for keyline irrigation at https://soilandhealth.org/wp-content/uploads/GoodBooks/The%20Keyline%20Plan.pdf