These are perhaps the weakest single item of the setup... they are buried, so they don't get good air exposure, and I have seen the heads of bolts just snap off due to corrosion. So I too am wondering what a better solution might be there too... bigger bolts?

I have the same square type carriage bolts. I am thinking of just going to hex head bolts, but they just don't seem as strong or as nice as the rounded heads (which have that thick square shoulder).

And of course going to some other bolts complicates installation of the chain plates.