When we walked the Central Route, my wife had significant knee problems. However, it wasn’t due to the hills. The cause of her knee problems were the cobblestones. Walking many miles through Portugal on the cobbles caused a great deal of knee stress and discomfort. She did not have any knee problems prior to the Camino.
A couple years later, we walked the Coastal Route. A night and day different experience for her. Few cobbles; no knee problems. So, from our experiences, I’d think the Coastal Route might be the one I’d choose if I had a knee problem. Even with that said, both are great Camino routes!
We crossed over from the coast at Vigo.