They have lasted a long time....I would wait for after the blooms die & fade off and it will take a 3 years period to get them all trimmed you can only take off 1/3 do that for 3 years straight, the dead stuff you can cut out...then use either a tar sealer or tar in a spray like Specricide.for when you do tree trimming to prevent critters from getting and making that their home............treat it with fertlizer, mulch and some lime around the bottom 20 inches from the roots it will rain in. The ones with only a few leaves on them, I would cut them all the way to the ground. They will come back stronger next year. With them being so big, you might needs to trim the top every year if you want it to grow thicker....just 3 inches. Be careful using tools. If it is over you head you might to have someone come in a do it for you.
When you trim the tops they will get thicker and create a privacy screen....You mentioned that they are on the property line.........if part is on your neighbors side you should mention that you want to trim & shape them on your side.....then maybe they will do their side and it will look even. PH should be 5.0 to 6.0