You can replace the RD with any fairly modern Shimano rear...
I don't know how common chain breakage is, but I haven't broken one on my Giant Sedona in the 2100 miles I have rode it... nor any other bike I have ever ridden.
How many miles have you rode the bike?
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Slow Ride Cyclists of NEPA
People do not seem to realize that their opinion of the world is also a confession of character.
- Ralph Waldo Emerson