IMO, it was asking for trouble when it was decided to use Toto toilets. Not because it's an inferior product,more so because there are so many models that finding correct parts can trip up owners and repairmen. Corky is good enough for most of the major brands and offer's flappers for 15% what Toto cost's. Are you positive the problem isn't with the people doing repairs?

Korky has great flush valves, which I prefer, however I am not a fan of their fill valves. Their design has flaws, better off going with a fluidmaster for fill valves.

If you still have problems with the korky and the flush valve, they need to use silicone grease on the flapper seal