Personally, I would create my own fork with this capability
and try to convince Wordpress to incorporate my changes.
How certain are you that you're going to find software that won't have similar problems? One of the reasons I like open source is that I have control and can make changes when necessary.
edit - also, you basically have customers that are saying they have a pain point, please solve it. I would tell them that Wordpress is designed for the majority and there may be edge cases that don't cover their particular usage. If they would like a customized solution, you can do it for them but it will cost x amount of dollars.
How certain are you that you're going to find software that won't have similar problems? One of the reasons I like open source is that I have control and can make changes when necessary.
edit - also, you basically have customers that are saying they have a pain point, please solve it. I would tell them that Wordpress is designed for the majority and there may be edge cases that don't cover their particular usage. If they would like a customized solution, you can do it for them but it will cost x amount of dollars.