Earlier quoted context omitted.
> "do unto others as you would have others do unto you. This one doesn't just have edge cases, it has a hole large enough to float an aircraft carrier through it. My grandparents (all deceased at this point) did not want to be treated in the same way I want(ed) to be treated. The much less absurd version is: "treat other people the way they would like to be treated ". Still edge cases, but the holes are much smaller.
Doesn’t work on the context of mental illness.
On the other hand, treating other people the way you wish to be treated is self-moderating. If I am greedy, the golden rule commands me to be generous to others. If I encounter somebody who's greedy, the golden rule only obliges me to give them whatever I would think myself entitled to.
The golden rule can't make everybody happy, and isn't supposed to. If there is some delta between the way your grandparents expect to be treated and the way you want to be treated, so be it. I don't think this is a reasonable objection to the golden rule.