What about writing her a letter and being honest about any mistakes you've made in the past and being accountable for them and offering a suggestion of a way forward - like meeting up together with a neutral third party (of her choosing, not the partner) or doing some family therapy sessions together?
I am a daughter who is NC with my mum, and the above is what I needed. In my case, it's my mum in an unhealthy relationship with a man who has not been willing to be accountable for the things she's done wrong (she facilitated contact by a convicted paedophile with my children). Every attempt to address my concerns with her has been met with excuses...'I know I was wrong to lie to you about that, but if I hadn't, I knew you would have cut me off sooner...' Everything is about her instead of about healing our relationship. I asked to do family therapy together, with a therapist she could choose, but she wouldn't, or would say she'd arranged it, but didn't and would back out at the last moment (while telling people it was me who wouldn't go!). What I needed was a willingness from her to admit what she did wrong and to move forward with getting support for both of us to heal our relationship. It was the evasion, sticking head in the sand, lying that made that impossible.
What I would say though is to let her know you are there and ready to work on things whenever she is and that you will be there for her anytime she needs you. But do not keep contacting her over and over again. One thing that really infuriates me is that my mum has not respected my wishes to be no contact (after my failed attempts above). She continues to contact me every birthday/mother's day/Christmas and it causes significant distress and anxiety - to the point that it's hard to enjoy Christmas with my children because I know I'll get a message from her telling me what a terrible person I am. Say what you need to say and leave the door open, but do respect her wishes for contact or not.
She is an adult and ultimately, she can make her own decisions, even if they aren't what you like. I wish that my mum hadn't gotten into a toxic relationship with a dangerous man and I wish that she would see the light and leave him and be the mum I want her to be. But she's a grown up. She can make her own decisions, even if I think they're bonkers. I can't control that and I can't force or manipulate her to be a better parent/grandparent. What I can control though is if I have her in my life and my children's lives (I won't) and how I move forward with my own life and live it the best way I can. Focussing on me and acceptance and healing myself has been life changing and helped me get 'unstuck' from the grief of our lost relationship.