Hi.
I am glad to hear you will be able to get a break. It sounds like you could all use one. I can feel the hurt and frustration you reference and I can relate.

I think this arrangement was an easy sell because I framed it as "We all just need a break" without telling my mom explicitly that the break is from her. This feels a little unsatisfying, though, because I am just so tempted to tell her (for the millionth time) the anger and hurt that is causing me to leave. But I know that this will likely ruin my plan for my sister because her defensiveness might make her retaliate by refusing to let her go.
You said that it feels unsatisfying to not tell her how you are feeling. I think that is something to take a close look at here. If your motivation is to make yourself feel better and get rid of excess frustration and hurt, will telling her accomplish that in a longterm and meaningful way for you?
I try to imagine these sorts of conversations with me being on the receiving end of such a message. How would you feel to be told such a thing? what would make it easier for you to hear and respond in a healthy way? What would make it harder? What factors are present in the current situation with your mom that would make the outcome good? And by good I mean other than speaking 'your truth' as a pressure release for pent up hurt and anger?
Don't get me wrong though. There are plenty of times when I had/have spoken up, not held back and have taken a stand, being "true to myself and my feelings". At the time it was all I knew and at the time and for a while later it felt good (for me). Years later I have learned more and changed and now my first question is usually "To what end?" and is this something that is in line with the person I want to be and is it consistent with my values. There are lots of variables that I consider, those are merely 2 of them.
Lets talk about it.