It sounds like you are doing really well!
My Ds are young, 4 and 9months, so we aren't having those conversations yet, whenever D4 does say something about Daddy I try to keep it focused on the validating. Two books that are really helping me is "The Power of Validation" and
I Don't Have To Make Everything All Better - Gary Lundberg and Joy Lundberg.
In my head if/when my daughter comes to me and says something like that I would say
"I'm hearing that you wish Dad wasn't grumpy. It can be hard having a grumpy Dad can't it"
"Yes and he doesn't do anything to help himself" (her)
"It sounds like you are feeling frustrated that he doesn't help himself get better"
"It is frustrating, if he did then he might not be grumpy anymore" (her)
"And if he wasn't grumpy anymore then things would be different in our house - I know it's hard honey" hug.
The "I don't have to make everything all better" book is helping me to just validate and not jump in and rescue. It's good in that it says stay in the moment and Validate what is going on there. Then if you want to revisit it the next day with your 'solutions to their problem', you can do so gently.
So after validating, I might the next day say "Hey honey, I was thinking about our conversation yesterday about Daddy, and it helps me to have hope when I think of how qualified the Drs are that Daddy is seeing" or whatever else I wanted to add at the time to make her 'all better'.
For me the rescue tendency is strong, and very hard to resist, especially for my Ds, of course I want to band aid them emotionally, I am scared that talking about it more will be more painful. But I am realising the power of just being heard, and I am trying to give my daughters that.
It's so great that your D has you to talk with, and to be a stable and peaceful presence for her!
Love Blazing Star