I think it's both water quality and substandard seals causing the disintegration. But no, it's not something you should need new toilets for. You can just replace the rubber washers.
Normally they're pretty generic, so you should be able to get what you need at any hardware store. It's possible Toto ones have something unique about them, but I doubt it. And I'd replace them on that toilet relatively soon; once they disintegrate enough, you're going to have a leak.
In terms of difficulty, it's not very hard or anything, but you do have to take the tank off. I'd watch some videos of that (Toto 2 piece toilets are basically the same as any others) to see if it's something you think you could tackle. And check out the manual for your toilet too, as that should explain the process of installing the tank. Which is basically what you'd be doing, first in reverse, and then in the normal order.
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