I really don't want to have to take the masts down again to replace the sheaves either. So I'm hoping that I can find a suitable solution, one that will last at least a few years before I have to rethink.
Just go up the mast to change the sheaves. Bring a line with you so a buddy can send up the tools.
Check out amsteel in 3/16th. $0.70 / ft and a min. strength of 2300 kg (4500lbs). Easy to splice, UV resistant cover, but I would definitely put a cover over the working/winch/clutch end. Slippery stuff otherwise.