Welcome. I think for any of us here, detaching from our adult troubled children is the most difficult thing we've ever done.....no matter what level of detachment we're at. You're not alone. This stuff is devastatingly hard. However, in your case, as in many of ours too, it is absolutely necessary.
In my opinion you're doing all the correct things, you're making good, healthy, positive choices for your own well being while allowing your daughter to face the consequences of her own behavior. Not easy to do. But if you and your daughter are to grow out of this unhealthy, patterned alliance, your choices may now facilitate that.
It's time for your daughter to grow up and it's time for you to have your own life and begin to enjoy your retirement years.
You might try reading the book, Codependent no more by Melodie Beattie. If you believe your daughter has mental illness issues, you might call NAMI, the National Alliance on Mental Illness, they can be assessed online and they offer excellent free parent courses which are very helpful with resources, information, guidance and support. If your daughter is a substance abuser, many parents find solace in 12 step groups like Families Anonymous, Al Anon and Narc Anon.
It's sad and unfortunate that your daughter finds it necessary to hold your grandkids as hostage over you, but that is a fairly common manipulative and cruel tactic used by our adult kids when they don't get what they want.
You've begun the challenging disconnection from your daughters choices and behaviors......you've developed the strength to make the necessary changes....... good for you. You are removing her from your home, you are not permitting her the use of your car and you are willing to do whatever it takes to get your life back......I say BRAVO. You've awakened from this enabling FOG (fear, obligation and guilt).....and now you're taking action.
This doesn't feel very good at the beginning, you're breaking old, dysfunctional behavior patterns....but stay the course, continue detaching, continue posting here, it helps. Keep yourself well supported. Put your needs and desires as the priority. You matter. You deserve a life of love and joy and peace. I'm glad you're here. You are not alone.