Oh, very very tall, so it will be a major job to cut them down if the neighbour decides to cut them down. I would let them know once a year of your offer to help take them out. You could offer to replace them with fruit trees hedgerow (for privacy) just on the inside of your property line and share the fruit with them or share or pay for a
fence for privacy. They probably like the privacy and are in no rush to cut down the trees.
In the meantime, you could grow some plants that like shade. Not knowing your ag zone and whether you are in a dry or abundant rain area it is hard to suggest which plants will grow. The plants may not like to the soil around the cedar so you may need to bring in some other soil on top of the ground soil like a
raised bed.
I live in a northern zone so I plant hostas in the shade. Young hosta leaves are delicious in salads or soups like spinach. They make great chop & drop material too.
In a food forest, in the shade of the cedars, you could set up a small bistro table & chairs so you can relax from time to time with a cup of ice tea. Or you could use this space for a garden nursery & work with large wooden table.