I put all new after market chains on last year 6G1633 this year I broke the right rear driver again and replace it with a Factory chain and it broke again.The mater link was sheared off .
I think I'd be looking very closely at the condition of the sprockets.
What about checking the chains for wear.? If the machine only does about 1000 hours a year it could probably be an annual check.
If the chains are stretching an inspection should hopefully detect that in advance of a chain breakage, although it would do nothing to detect a master link failure. How many of the failures have been a broken master link.?
A bit oif a curve ball from left field but have you inspected for wear in the fibre bushes located in the pivot between the tandem and the final drive.?
If you have a ripper it's easy enough to jack up the machine using that and the moldboard. If not then you have to put a jack under the chassis. Watch for the tandems "falling down" as you raise up the machine. Wear in that bushing can affect the way the chains run big-style.